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It all did start with the ninety-five theses, in a sense. Luther probably did not actually nail them to the church door—at least no one at the time tells us so. And if he did, it was not in anger or protest against the church. He was trying to arrange an academic discussion, and evidently that’s where the bulletin board was. What we do know is that he mailed them off to his archbishop, together with a treatise on indulgences and a cover letter dated October 31, 1517, so that is the date remembered as the beginning of the Protestant Reformation.

Martin Luther was a professor at the University of Wittenberg, which is why he was arranging a disputation, a standard medieval form of academic discussion that would subject his theses to helpful critical debate so that he could clarify his own position. (He was certainly not wedded to all ninety-five theses.) He was also the official town preacher with pastoral responsibilities for the laity of Wittenberg, which is why he was trying to clear up some issues concerning the theology of indulgences, which were being sold very aggressively in neighboring territories, much to the harm of the souls in his charge. And he was a pious monk, intensely obedient to authority, who was convinced the pope could not possibly approve of turning indulgences and the forgiveness of sins into a kind of merchandise at the expense of Christ’s people.

What Luther did not know at the time is that the pope and the archbishop were the ones profiting from this merchandise, each claiming half of the take. So it is not surprising that events took a turn he did not anticipate. Within five years, this intensely obedient monk had concluded that obedience to God precluded obedience to the pope, and a schism in the Church followed. Five hundred years later, it is possible to set aside the economic and political factors leading to the schism and ask specifically about the theology that first took shape in those five years at the beginning of Protestantism. Must it divide us, or does it have something of value to say to the whole Church? The intensive labor of ecumenical discussions since Vatican II has made it possible to imagine a positive answer to this question. Indeed, the most serious challenges in Luther’s theology may be to the Protestant tradition.

At the starting point in 1517, Luther’s pastoral concern was unfamiliar and hardly Protestant. He thought indulgences made penance seem much too easy, undermining the lifelong work, required of all Christians, of contrition, which he identified with heartfelt self-hatred. True inward penitence meant condemning ourselves, in obedience to the Word of God accusing us. It was a grim and harrowing spirituality, which in hindsight looks like all law and no Gospel, as Lutherans would now put it. In that sense Protestant theology had not yet begun in 1517. Something else had to happen first. Luther had to find a Word of God that was not an accusation but a bestowal of grace and forgiveness.

What is often overlooked is that he found it in Catholic sacramental theology. In 1518, as the ninety-five theses were widely published and the controversy over indulgences exploded, Luther had to learn to think sacramentally for the first time. Indulgences concerned the effects of penance, so in order to explain his theses, Luther had to account for the role of the sacrament of penance. Judging by the writings we have from this period, he had never thought seriously about any of the sacraments before.

The result surprised everyone. He ended up insisting on an affirmation of sacramental efficacy that was stronger than anything his papal critics could accept. By the end of 1518, he was teaching that Christians hearing the word of absolution in the sacrament of penance (“I absolve you of your sins in the name of the Father, the Son, and the Holy Spirit”) should simply believe their sins are absolved. Contrary to the long-standing teaching of the medieval Church, they should not concern themselves with whether their inward penance or contrition is sufficient to confirm the absolution and obtain forgiveness of sins. The promise of Christ in the Gospel is sufficient by itself and needs only to be received in faith. We should be quite certain our sins are absolved and forgiven, Luther began to teach, because Christ’s promise of the keys (“Whatever you absolve on earth shall be absolved in heaven,” Matt. 16:19) means that to doubt the word of absolution is to call Christ a liar.

If you want to blame Protestantism on Luther’s peculiar personality, this is where to look. Faith comes by hearing, says the apostle (Rom. 10:17), and Luther is the greatest and most obsessive theologian of hearing in the Christian tradition. For Luther, everything depends on hearing the Word of God, taking hold of it, clinging to it, and not talking back—not calling God a liar. His treatment of the doctrine of justification always turns on the conviction that “God is true, though every man be a liar” (Rom. 3:4). Everything depends on God being true to his word and keeping his promise, which the deceitfulness of our unworthy hearts cannot overturn. Supporting this conviction is an imagination that is utterly auditory, not visual. The illustrations and examples in Luther’s writings are not about how to see things but how to hear them. We often find him composing mini-dialogues, with words put in the mouth of God, Christ, the devil, or our own conscience.

What happens in 1518 is that Luther’s fierce conviction that we must never talk back against God’s word meets the medieval doctrine of sacramental efficacy, and results in the Protestant conception of the power of the Gospel. The term “Gospel” has a very distinctive meaning in Protestant theology. It does not just designate the four books of the New Testament that go by that name. It is the Word of God with power to save us because it gives Christ, his forgiveness and grace and righteousness, to all who put their faith in it. Hence for Luther, “Gospel” includes any scriptural telling of the story of Christ given for us, by any of the prophets or apostles, such as Isaiah announcing, “For unto us a child is born,” or Paul teaching that nothing can separate us from the love of God that is ours in Christ Jesus. And because the Gospel in this sense tells us the truth about Christ given for us, it is in effect an external means of grace just like a sacrament, because it gives us what it signifies.

In fact, in a sermon on Christmas Day, 1519, Luther comes right out and says the Gospel is a sacrament. It has the same structure and operation that Catholic theology finds in all the sacraments, which are external signs efficaciously conferring the grace they signify on those who properly receive them. Luther simply adds: The proper reception of the word of the Gospel is faith alone. Hence in the sermon he says that “all the Gospel stories are a kind of sacrament, that is, sacred signs through which God brings about, in those who believe, whatever the story designates.” This is why the story of Christ differs from every other history we might study. Even in the four gospels, the life and deeds of Peter, John, or Mary offer us only examples of righteousness and virtue to follow, whereas the story of Christ actually gives us the righteousness, virtue, and salvation it signifies, just as baptism gives us new birth, because what we receive by believing this story is Jesus Christ himself. On Christmas Day we can come to Bethlehem and find a tender maiden with a baby on her lap and say, “Mother, this baby is mine also.”

Luther’s doctrine of justification by faith alone thus depends on a sacramental notion of the Gospel. It means we take hold of Christ and make him ours by receiving the Gospel the same way we receive a sacrament, not doubting that it confers what it signifies—for what it signifies is nothing less than Jesus Christ. Simply by believing this word, we are united with Christ himself, God in the flesh, and thus receive all that is his, like a bride inheriting every good thing that belongs to her husband. Luther speaks of a wondrous exchange, by which this bridegroom gives us his righteousness, holiness, and eternal life, while we have nothing to give him in return but our sin, death, and hell. That is why he has a mighty battle to fight on the cross, where he defeats every evil for us and for our salvation, as Luther explains in one of the first great treatises of Protestant theology, The Freedom of a Christian (1520).

After first being given Christ himself, and secondly every good thing that is his (grace, righteousness, holiness, etc.), a third step is needed to complete our justification by faith alone. It is the least fundamental but still necessary. Because all Christians are imperfect and still sinners as long as they remain on the road of this mortal life, as Augustine taught long ago, they need for their sins not to be counted against them. The non-imputation of sins thus becomes the “forensic” element in Luther’s doctrine of justification, where language from the law court or forum (from which comes the technical term “forensic”) has a place in describing the salvation that is ours in Christ, not as the basis of justification but as a necessary stopgap until Christ and his righteousness are fully formed in us.

A careful reading of the founding works of Protestant theology by Luther poses some serious challenges to later Protestantism. Consider each of the preceding three steps in turn.

Most fundamental is the first step, Luther’s teaching that by faith alone we are united with Christ. Protestants love to talk about “accepting Christ by faith,” which certainly owes a great deal to this fundamental teaching of Luther’s. Usually, however, it is presented as a decision we must make, as if it were by our own free will. Luther, by contrast, hates the very idea of free will when it is applied to matters of salvation, for our confidence in our own free will lies at the core of our efforts to be justified by good works rather than faith alone. The great pastoral aim of Luther’s doctrine of justification is to free us from the kind of performance anxiety that arises whenever our salvation depends in any way on us, our hearts, our will, or our doings. For anything we do is something about which we can ask, “Am I doing it well enough?” And for Luther the answer is always “not well enough to save you from damnation.” No act of our free will, and hence no decision of ours, is an exception to this rule.

In particular, no decision of ours is untainted by our persistent unbelief, which Luther identifies as the root cause of the sin that is still in us. The sin of unbelief is something we spend a lifetime learning to repent of. We are commanded to repent, believe, and be baptized, and once baptized we simply have no excuse for our residual habit of living and thinking as if Christ were not our beloved savior—as if he did not die for our sins and were not raised for our justification, did not give himself to us in baptism and the Lord’s Supper, and has not bestowed his Holy Spirit and faith upon us.

We have no decision to make about this. God has already promised these things to us in his word, and he has bestowed them on everyone who is baptized by addressing each one of us in the second person singular, saying “I baptize you in the name of the Father, the Son, and the Holy Spirit.” Here “I” really means Christ, who speaks through the mouth of the minister, and “you” means me in particular—whoever is addressed in the particular utterance of this sacramental word at a particular time and place. So to think I must make a decision about whether I belong to God, Father, Son, and Holy Spirit, is to doubt God’s word given to me in baptism, and in effect to call God a liar. Of course I do make decisions for Christ all the time, but not a single one of them is worth trusting as the basis of my salvation, because my faith is always at least partly unbelief, since I am still a sinner like every Christian.

Unbelief thus remains a perpetual problem in the Christian life, and it is important for us to feel our helplessness as we are assaulted by a host of temptations not to believe, which Luther famously calls Anfechtungen or “assaults,” and regards as an essential experience of the Christian life. We would be fools to fight these assaults by trusting in our own free will or ability to believe. For Luther, we must believe that we are Christians because Christ said so in our baptism, not because we have made a decision or had a conversion experience or done something to make ourselves into believers. If asked whether we are truly Christians, the answer Luther teaches us to give is simply “Yes, I am baptized.”

In that sense Luther’s faith is deeply unreflective, unlike most later forms of Protestant theology. When Luther teaches justification by faith alone, he is not requiring us to put faith in faith. On the contrary, he wants us putting faith in the Gospel alone. Any account of faith that focuses on the experience of faith—any theological turn to subjectivity, such as in liberal theology—has missed Luther’s point. What we experience, for the most part, is our own sin and unbelief. Faith means turning away from our experience to take hold of Christ alone by believing, against all doubt and temptation, that what the Gospel tells us about Christ given for us is really true.

Justification by faith alone is thus justification by Christ alone. This has everything to do with hearing the Gospel spoken aloud in external words, through which Christ claims each of us by saying “you” in a way that includes me. When I hear “This is my body, given for you,” spoken in the proper liturgical setting, it is sheer unbelief to deny that Christ is mine. Conversely, it is only by believing in such a word addressed to me that I know that I am a Christian, that I have a savior, that I am an heir of eternal life—not because I know I believe (frankly, this is not something I know) but because I know Christ does not lie. Who I am in Christ takes shape from the truth of such external words. They have sacramental efficacy as a means of grace. In this way Protestant theology needs a Catholic notion of sacrament in order to carry out its deepest intention, which is to put faith in the Gospel of Christ alone.

In the second step, Luther focuses famously on the righteousness of God, the justitia Dei that is the basis of his doctrine of justification. What is often overlooked by later Protestant theology is that Christ’s righteousness is the righteousness of God. Recently a strong Finnish tradition of Luther scholarship has repaired this oversight and drawn the appropriate conclusion: that Luther’s teaching about union with Christ, followed by the wondrous exchange in which Christ shares with us every good thing that is his, implies a doctrine of deification. For the goods he shares with us include all that is divine in him, in which we participate—as the Church Fathers say—not by nature but by grace. In Luther’s terms, every divine gift is ours in Christ, who is ours by faith alone.

This means that the third step in our justification is merely God not counting our remaining sins against us. Luther has no need of the much more elaborate forensic doctrine that arose in later Protestant theology, according to which justification consists in God imputing Christ’s merits to us, like a kind of credit transferred to our account. Merits are something human beings earn, and no doubt Christ, being true man as well as true God, has a boatload of them. But Luther follows Augustine and Paul in insisting that our justification is based on the righteousness of God, who is far above the task of earning merits.

All told, I think such a sacramental reading of Luther, which is also a historically accurate reading, poses a greater challenge to most forms of Protestantism than to Catholicism. Still, Luther poses serious challenges to Catholic thinking that Catholics might grapple with profitably.

In his 1520 treatise The Babylonian Captivity of the Church, Luther became famous as a critic of what later Protestants called “the Catholic sacramental system.” But in fact his criticism was meant as a defense of the sacraments. He treats four of the seven sacraments (marriage, ordination, confirmation, and anointing of the sick) as rites approved in the Bible but not properly called by the name “sacrament” because they are not based on a promise of Christ himself. Penance he continues to regard as a necessary sacramental practice, but not a separate sacrament from baptism. It does not count as a separate sacrament because it does not have a sign attached to it in addition to the word. Moreover, the Trinitarian form of the word of absolution, echoing the baptismal word, illustrates Luther’s teaching that all repentance is really a way of returning to our baptism.

When it comes to the Eucharist, Luther does not simply reject transubstantiation but regards it as a permissible theological opinion that he personally finds implausible. So long as Christ’s body and blood are really present in the sacrament, what need is there for the extra miracle of doing away with the bread and wine and making them into mere appearances? His disagreement with Catholic teaching on this score is shallow compared to his deep and vehement rejection of Protestant theologies that deny Christ’s body is literally present with the bread in our mouths and pressed with our teeth. When asked in later years whether this means Christ is chewed up and consumed, Luther explicitly agrees with Thomas Aquinas’s answer: The body of Christ is always present whole and undivided, not cut into parts and digested like ordinary nourishment. It is Christ’s glorious, life-giving flesh, not perishable food.

Luther’s really serious challenges to Catholic thinking are philosophical. This is the kernel of truth in the largely misguided accusation by many Catholic historians that Luther’s theology is an outgrowth of nominalism, the late medieval philosophy that denied the metaphysical reality of essences. It is true that Luther was educated in the nominalist tradition—together, it should be said, with a characteristically late medieval version of the Aristotelian philosophy of nature. But his theology is not an outgrowth of any philosophy but rather of his devotion to the Word of God. If Luther’s theology is something you don’t like, call it a desperate and obsessive devotion, but don’t make the mistake of thinking it is merely the reflection of a philosophical position.

Nominalism only matters for Luther because it does not obstruct or distract from his obsession with the word, as a more robust metaphysics might. In the same vein, Aristotelianism matters for Luther because its concept of form gives him a language for talking about Christ being formed in our hearts by the hearing of faith. In Aristotelian accounts of perception, the same form is in the mind as in the thing perceived, like the form of a favorite prayer or song or story that is both resounding in the air and learned by heart. A nominalism that cannot accommodate even this level of Aristotelian forms or essences is too restrictive for Luther’s purposes.

What Luther jettisons is a heavily metaphysical spirituality that aims for a beatific vision of the essence of God, taking this to be the goal of the Christian life. Luther accepts the traditional metaphysical attributes of God, such as eternity, immutability, omnipresence, and so on, but these are not objects of his devotion. Above all, he has no use for the rich notion of intellectual vision shared by Augustine and Aquinas, going back ultimately to Plato’s metaphor of seeing the supreme Good with the mind’s eye. Luther does not aim to see God’s essence but to hear him speak, for it is in his word that God gives himself to be known.

This is a deeply unphilosophical notion, in that it gives up the project of seeing the ultimate Truth for ourselves and relies on a kind of secondhand knowledge based on the authority of prophets and apostles and Christ himself. In that sense the knowledge of God is opaque, dependent on what others tell us rather than our own reason and insight. This dependence on the word of another—you could call it an epistemology of hearing—is at the root of the anti-metaphysical streak that frequently shows up in Protestantism, which makes it more at home in modernity than Catholicism.

The question for Catholics to consider is whether the priority given to hearing is actually fitting, more theologically sound than their usual ways of thinking. If God is a person rather than a principle, coming to us in the person of his own Son, isn’t believing what he has to say about himself the deepest and most appropriate way to know who he is? While we might be able to figure out some people, like the way we can sometimes see through a liar, we cannot know people who are wiser and better than us if we are unwilling to listen to them, learning and trusting what they have to say about themselves. Vision only comes in at the end, when we see that they are true to their word. Thus we can hope ultimately to see the coming of the kingdom of God, but this will involve something like what Julian of Norwich hopes to see when she hears Jesus promise, “All shall be well, and all manner of things shall be well, and you shall see yourself that I shall keep my word and make all things well.”

Half a millennium after the ninety-five theses, Christians outside Protestantism may begin to receive Luther’s challenges as a gift, presenting a piety of the Word of God that enriches the whole Christian tradition. The Joint Declaration on the Doctrine of Justification, published by the Roman Catholic Church and the Lutheran World Federation at the turn of this century, arrived at a theological consensus that Luther’s doctrine of justification by faith alone need not be Church-dividing. The underlying recognition was that justification by faith alone amounts to justification by Christ alone—and on that, both sides could agree. This makes it possible to accept the challenge of embracing the Gospel as the indispensable external means of grace, the source of every sacrament, and the meaning of every icon.

Most important, Luther’s challenge affects Christian life by freeing Christian love to be love, removing the kind of performance anxiety that makes it about ourselves. If we are justified by faith alone, then works of love serve our neighbors, not our desire to justify ourselves or make ourselves holy. We are holy because Christ claims us as his own through word and sacrament, and Christian love takes root in our hearts because we believe what Christ has done. Luther challenges us to recognize that from the beginning of the Christian tradition, this is always how Christ has saved his people, sanctifying them and giving them to their neighbors in love. Thus justification by faith alone is not new, even though Luther’s formulation of the doctrine is.

How we have always been justified by faith alone is best seen in light of Luther’s distinction between law and Gospel. Both the law of God and the Gospel of Christ are God’s word, but the former only gives us instructions while the latter gives us Christ. For the law tells us what to do, but the Gospel tells us what Christ does. The distinction grows out of Augustine’s insistence, in his great treatise On the Spirit and the Letter, that telling us to obey the law of love does not help us do it from the depths of our hearts; only the grace of Christ can give us such a heart. Luther merely adds: The place to find the grace of Christ is in the Gospel of Christ.

A great many preachers, Protestant as well as Catholic, overlook the distinction between law and Gospel, thinking they can change people’s lives by giving them practical advice—as if telling them how to be inwardly transformed could help them do it. Augustine already knew better. Luther’s addition to Augustine’s insight is merely the glad recognition that there is indeed something preachers can do to help us be transformed: Instead of advice, they can give us Christ. 

Phillip Cary is Scholar-in-Residence at the Templeton Honors College at Eastern University.