Augustin explains that at some point he changed his view from synergism to divine monergism in salvation. He argues that due to our fallen state, we are not only partly dependent upon Christ for our conversion but totally dependent upon Christ.
"that we must boast in nothing, since nothing is our own." And in order to show the, he appealed to the apostle as a witness, where he said, "For what hast thou that thou hast not received ? And if thou hast received it, why boastest thou as if thou hadst not received it?" And it was chiefly by this testimony that I myself also was convinced when I was in a similar error, thinking that faith whereby we believe on God is not God's gift, but that it is in us from ourselves, and that by it we obtain the gifts of God, whereby we may live temperately and righteously and piously in this world. For I did not think that faith was preceded by God's grace, so that by its means would be given to us what we might profitably ask, except that we could not believe if the proclamation of the truth did not precede; but that we should consent when the gospel was preached to us I thought was our own doing, and came to us from ourselves. And this my error is sufficiently indicated in some small works of mine written before my episcopate. Among these is that which you have mentioned in your letters wherein is an exposition of certain propositions from the Epistle to the Romans. Eventually, when I was retracting all my small works, and was committing that retractation to writing, of which task I had already completed two books before I had taken up your more lengthy letters,--when in the first volume I had reached the retractation of this book, I then spoke thus:--"Also discussing, I say, 'what God could have chosen in him who was as yet unborn, whom He said that the elder should serve; and what in the same elder, equally as yet unborn, He could have rejected; concerning whom, on this account, the prophetic testimony is recorded, although declared long subsequently, "Jacob have I loved, and Esau have I hated,"' I carried out my reasoning to the point of saying: ' God did not therefore choose the works of any one in foreknowledge of what He Himself would give them, but he chose the faith, in the foreknowledge that He would choose that very person whom He foreknew would believe on Him,--to whom He would give the Holy Spirit, so that by doing good works he might obtain eternal life also.' I had not yet very carefully sought, nor had I as yet found, what is the nature of the election of grace, of which the apostle says, ' A remnant are saved according to the election of grace.' Which assuredly is not grace if any merits precede it; lest what is now given, not according to grace, but according to debt, be rather paid to merits than freely given. And what I next subjoined: ' For the same apostle says, "The same God which worketh all in all;" but it was never said, God believeth all in all ;' and then added, ' Therefore what we believe is our own, but what good thing we do is of Him who giveth the Holy Spirit to them that believe: ' I certainly could not have said, had I already known that faith itself also is found among those gifts of God which are given by the same Spirit. Both, therefore, are ours on account of the choice of the will, and yet both are given by the spirit of faith and love, For faith is not alone but as it is written, ' Love with faith, from God the Father, and our Lord Jesus Christ.' And what I said a little after, ' For it is ours to believe and to will, but it is His to give to those who believe and will, the power of doing good works through the Holy Spirit, by whom love is shed abroad in our hearts,'--is true indeed; but by the same rule both are also God's, because God prepares the will; and both are ours too, because they are only brought about with our good wills. And thus what I subsequently said also: ' Because we are not able to Will unless we are called; and when, after our calling, we would will, our willing is not sufficiently nor our running, unless God gives strength to us that run, and leads us whither He calls us;' and thereupon added: ' It is plain, therefore, that it is not of him that willeth, nor of him that runneth, but of God that showeth mercy, that we do good works'--this is absolutely most true. But I discovered little concerning the calling itself, which is according to God's purpose; for not such is the calling of all that are called, but only of the elect. Therefore what I said a little afterwards: ' For as in those whom God elects it is not works but faith that begins the merit so as to do good works by the gift of God, so in those whom He condemns, unbelief and impiety begin the merit of punishment, so that even by way of punishment itself they do evil works'--I spoke most truly. But that even the merit itself of faith was God's gift, I neither thought of inquiring into, nor did I say. And in another place I say: 'For whom He has mercy upon, He makes to do good works, and whom He hardeneth He leaves to do evil works; but that mercy is bestowed upon the preceding merit of faith, and that hardening is applied to preceding iniquity.' And this indeed is true; but it should further have been asked, whether even the merit of faith does not come from God's mercy,--that is, whether that mercy is manifested in man only because he is a believer, or whether it is also manifested that he may be a believer? For we read in the apostles words: ' I obtained mercy to be a believer.' He does not say, ' Because I was a believer.' Therefore although it is given to the believer, yet it has been given also that he may be a believer. Therefore also, in another place in the same book I most truly said: ' Because, if it is of God's mercy, and not of works, that we are even called that we may believe and it is granted to us who believe to do good works, that mercy must not be grudged to the heathen;' although I there discoursed less carefully about that calling which is given according to God's purpose."
- Augustine, A TREATISE ON THE PREDESTINATION OF THE SAINTS chapter 7 [III.]