For Agamben ( Opus Dei: An Archaeology of Duty ), Suarez’s De natura et essentia virtutis religionis is a crucial moment in the development of modern notions of duty, particularly in the use to which debitum is put: “The concept of debitum , which in Aquinas is hardly formulated, becomes first of all the formal definition of religion and the nucleus around which the entire treatise revolves” (104-5).

In defining religion, Suarez appeals to Isidore and Augustine, but also to Lactantius, who defined religion by reference to the judicial notion of vinculum , a ligament or line that binds two things together. From this, he offers a definition of religion that links duty and habit: “the name of religion can thus be correctly explained: since the rational creature is bound by a natural debt ad by an intimate inclination to offer worship to its author, it is bound anew by a voluntary choice and by a habit added to it. Therefore the virtue that infills this officium can be called religio .” Religion is not simply the fact of worship but the fact that worship is rendered to God as fulfillment of a debt (105). Again, Agamben is interested in the ontological import: “In the idea of a being that is totally dissolved into a debt, into a having to be, law and religion necessarily coincide” (106).

Paul says, “owe no one to anything.” In the light of this, Suarez’s reintroduction of debt into the heart of ethics is a major reversal of Paul’s evangelical. For more, see my forthcoming book on Gratitude .