Bright young ladies, both excellent students at their respective excellent schools, my seventh grade catechism students pay attention, ask good questions, and remember interesting little facts like “Hildegard of Bingen was a twelfth century mystic and writer.” But even I was surprised when they told me they completely understand the Incarnation.

Their textbook breaks the Apostles’ Creed into its twelve statements of faith. We got all the way to number seven, “There are two natures in Jesus: divine and human,” when one of my precocious pupils interrupted. “I totally get it,” she explained. “It’s easy.” I was skeptical, especially as point number eight notes that the Incarnation is a mystery. I asked her to explain.

She drew me a Venn diagram. The outermost circle represented divine nature and inside it she placed three circles to represent the three persons of the Trinity. She inscribed “human nature” inside the second person. “Manichean!” I thundered, slamming my fist on the table. “Let her be expelled who has the mad idea that the servant-form Christ took from us is of a heavenly or some other kind of being!”

Not really. But I did note that in her illustration, the “human” nature of Christ was in fact “divine nature.” And as the Manicheans noticed, if the “human” nature is really divine, what looked like a human nature in the gospel accounts of Jesus was just an appearance. Jesus walking, eating, weeping—it would all be just an illusion. This leaves a Manichean, St. Thomas notes, in a tough spot. For the gospel doesn’t say that Jesus appeared to walk, eat, or weep; it says he did those things. And if Scripture errs on this matter, how can we trust it on any other matter?

But my students are not the sort to be deterred by one little charge of heresy. So armed with their newly heightened awareness that the Incarnation unites two natures—one fully human and one fully divine—they were determined to grasp just how those two natures are united.

“It’s like cookie dough” one of the girls explained. “You have sugar cookie dough and chocolate cookie dough and when you combine them and bake it you have . . .” but before I could bellow, “May your tongue and mind which have formed such blasphemy be burned up by divine fire!” she had realized her own mistake. If you combine sugar cookie dough and chocolate cookie dough, you get neither a sugar cookie nor a chocolate cookie.

She had resurrected the error of the heresiarch Eutyches. If the Incarnation, the union of God and man, were a union in the nature, we would have neither God nor man as a result. We would have to admit that divine nature had changed, and if it were capable of change, it wouldn’t be divine nature: “For I the Lord do not change” (Malichi 3:6).

So we tried again. And this time the mistake was mine. I should have stuck with Aquinas, in trying to offer them an analogy, and talked about the soul’s relation to the body, but instead I talked about the sole’s relation to a shoe. “Take your foot. You could wear a ballet shoe or a tap shoe and, depending on the shoe you’re wearing, the motion of your one foot will be expressed differently,” I said, and their faces lit up with comprehension. “I get it!” exclaimed one of the girls, “the Divine person who is not distinct from his nature puts on human nature when he wants to do human things like going to weddings and eating fish!”

I was hatching a nest of Nestorians. If Christ puts on human nature like we put on shoes—or as Nestorius would have it, like a God dwelling in his temple—his union with human nature would not really be a union at all. And from there it’s, just a hop, skip, and a jump to claiming that the Incarnation, in fact, has two persons and then it would not be true that the “Word became flesh,” and we would be left, once again, wondering why Scripture deceived us.

“No, no!” I quickly qualified. “It’s like two shoes you wear on one foot at the same time and you never take them off and they’re fused to your foot.”

“Disgusting!” exclaimed one of the girls. “Do you even know what you’re talking about?”

“No!” I wanted to retort.

“This is impossible!” cried the other.

“You’re right!” I was ready to agree, with just a minute or so left of class and two very disgruntled seventh graders staring me down from across the table. Then my former Manichean countered, “We know it’s not impossible because it happened.”

After an hour of echoing various heretics, she was, more or less, paraphrasing St. Thomas, who offers these encouraging words in the first question of the first part of the Summa: “Since faith rests on infallible truth, while it is impossible for something contrary to the truth to be demonstrated, it is manifest that the arguments which are made against faith . . . can be answered.”

It was the beginnings of the right disposition of the student to the subject of theology. We have, in one sense, the greatest certainty of that which we know by faith because its principles are given to us by God and are about him who never errs and never changes. So we can confidently probe what is divinely revealed and seek to understand because, as my seventh grader knew, “It’s not impossible because it happened.”

But as my very bright and very frustrated students began to realize, we’re not going to “get it” like we get that “Hildegard of Bingen was a twelfth century mystic and writer.” The reality divinely revealed to us, who we try to explore in theology, is above reason, though not irrational.

As Pope Benedict explained it in The Nature and Mission of Theology, “Knowledge never transfers this reality into a constitutive element of my own thought, but rather the converse is true: It is I who make myself over to it, while it always remains above me.” It is like a spring that does not fail. We can’t take it away with us but can only keep coming back to it to refill our tiny buckets—the always insufficient words we use to dip into the mystery.

As we go back again and again to the spring, the words multiply and we gain a little in understanding: From “the Word became flesh” we extract two nature’s united in one person who is consubstantial with the Father and Holy Spirit. We say “God is man” and “Man is God”; we can say “God became man” but not “Man became God”; we cannot say “Christ is creature,” but of “Christ, as a man,” you can predicate “creature.”

It’s not a perfect understanding, but it is some reflection of truth. Our mistake is in refusing to make ourselves over to the whole truth, choosing instead the little pieces that fit a theory we can fully grasp. It is the very definition of heresy: the obstinate denial by a baptized Christian of some truth of the faith. And it’s the mistake we fall naturally into when we insist that “It can only have happened if we can see how it is possible,” rather than “We know it’s not impossible because it happened.”

Meghan Duke is an assistant editor at First Things.