As Augustine himself makes clear at the beginning of Book 15, in his own summary of the fourteen preceding books at DT 15.3.5, the first four books of De Trinitate are devoted to establishing the scriptural foundation for the Doctrine of the Trinity. It is especially important to Augustine to make clear in these books that the Son, the second person of the Trinity, is not “less than He who sends, because the latter sends and the former is sent, since the Trinity, which is equal in all things, and is also equally unchangeable in its nature, invisible, and present everywhere, works inseparably” (DT 15.3.5).

The next three books, Books 5 through 7, are more philosophical than what had preceded. They are aimed at developing a philosophical, and especially a metaphysical, vocabulary for talk about God. Thus in Book 5 Augustine insists that “nothing can be said of God according to accident, because nothing accidental can happen to Him” (DT 5.5.6). Indeed, he goes on, there is nothing changeable in God at all. Yet, he adds, we may say some things about God relatively, or in relation, “as the relation of Father to Son, and of Son to Father.”