Answer to Maximinus
This text is the theological equivalent to a 'rejoinder on the stairs'. Augustine, seemingly bested in a debate with Maximinus, writes this tract in order to continue the debate, and show the points in which he is correct, as well as to highlight arguments Maximinus had no answer for and simply avoided.
The text comes in two books, the first concerns points that Maximinus could not refute, the second refutations of Maximinus by Augustine.
Maximinus shows himself firmly in a Latin tradition deriving from Tertullian and Novatian, in that he reads OT theophanies as Christophanies, but then extends this to insist that the Son is visible, the Father alone invisible, and to drive a wedge between their common nature. Augustine employs several arguments to force admittance that the Son is invisible in his nature, though he does appear, and that it is not a characteristic of the Greater to be unable to be seen by the lesser. Augustine denies there are any texts that show the Holy Spirit adoring the Father as if a creature. He also employs a sophisticated exegesis of Jn 17 and like passages, to show that the unity of disciples in Christ is as not identical the union of Father and Son. Augustine's primary tactic in that point is to distinguish unity asserted absolutely, and unity asserted in respect to something. To Maximinus' argument that: if the Son is the same as Father, then the Son is unborn as the Father, Augustine simply responds with the example of Adam: unbegotten, as made by God, yet begets a son of his own nature.
In Book 2, Augustine moves to Maximinus' arguments. The key question is “whether the Father and the Son and the Holy Spirit have a different substance, as your say, or rather, as we say, have one and the same substance and whether the one God is the Trinity”; Sabellianism is not in view.
Augustine demonstrates his commitment to partitive exegesis, as for instance in dealing with Phil 2:6-9, which he understands economically where Maximinus understands absolutely. Augustine lays down some clear statements in 2.10 where he writes, “The Father and the Son and the Holy Spirit are three persons, and because these three have one substance, they are supremely one without any difference of natures or of wills” and “Christ is one person with a twofold substance, because he is both God and man.”
There is an interesting point in 2.22, as they debate 1 Cor 3:16, then 6:19-20. Maximinus wants to use the Spirit's cleansing as sign that he prepares the way for God, and so is not God. Augustine brings in other texts such as 1 Cor 6, to argue that the Spirit is himself the indweller of this temple, and thus God.
In 2.26 onwards, they return to debating the Old Testament. Augustine shows that he is not willing to read this simply as Christophanies, but as Theophanies without a clear distinction of person, using creaturely or corporeal forms to effect a manifestation of presence. If the Father is invisible and spirit, so too the Son. The visibility of the Son is inadequate to support OT Christophanies.