FOTC 86:218–22. Cf. gr. et pecc. or. 1.31.34 (CSEL 42:152).
he used to devastate and had been angrily turned against—he was suddenly converted to it by a more powerful grace. He was converted by the One to whom the prophet said, just as it was about to happen: "You will turn us and bring us to life" (Ps 84:7). So he not only went from being one who refused to believe to becoming a willing believer, but even from being a persecutor to becoming one who suffered persecution in defense of that faith he once used to persecute. It was a gift given him by Christ: not only to believe in Him, but also to suffer for Him.
(2.5) And therefore commending this grace—which is not given according to merits, but rather causes all good merits—he says: "Not that we are sufficient to think anything as from ourselves, but our sufficiency is from God" (2 Cor 3:5). Let them consider this well and weigh these words, those who believe that the beginning of faith is our doing and that only the supplementing of faith is from God. For who would not see that thinking comes before believing? For no one believes anything unless one has first thought that it is to be believed. However hastily, however speedily, some of our thoughts fly before the will to believe, and even if this will follows them in such a way that it appears to accompany them, as though they were inseparable, still it is necessary that all things that are believed are believed after thought has preceded. Yet even to believe is, in fact, nothing other than to think with assent. For not everyone who thinks believes, for many think in order that they may not believe, but everyone who believes thinks, and in believing, thinks, and in thinking, believes. So in what concerns religion and piety (of which the Apostle was speaking), if we are not "sufficient to think anything as from ourselves but our sufficiency is from God" (2 Cor 3:5), it follows that we are not capable of believing anything, as though from ourselves, for we cannot believe anything without thought, but "our sufficiency," by which we begin to believe, "is from God." Therefore, just as no one is sufficient to himself for the beginning or the completion of any good work (which these brothers, as your letters indicate, already believe is true), so that in the beginning as well as in the perfecting of every good work, our sufficiency is from God, so no one is sufficient to himself either to begin or to perfect faith, but "our sufficiency is from God." For without thinking, there is no faith, and we are not "sufficient to think anything as from ourselves, but our sufficiency is from God."99
99. De praedestinatione sanctorum 1.1–2.5 (BA 24:468–74); my trans., based on Mourant and Collinge, FOTC 86:218–22. Cf. gr. et pecc. or. 1.31.34 (CSEL 42:152).