I think efforts to portray Augustine as a proponent of Calvinistic determinism on predestination and free will are overblown. He affirmed the absolute necessity of both God's grace and free will.
Like most of the Fathers, his many, many writings have often been taken out of context and used to justify later heresies.
The first Father to be claimed by the Calvinists, of course, was the Apostle Paul. Doesn't mean he was, no matter how many times they spin his words in the epistles.
It has been well established that Augustine presented a distorted view of grace and human will...Even though he is a doctor of the Catholic Church even so there his errors are acknowledged and corrected...So also his disciples Prosper of Aquitaine and later Thomas Aquinas rejected many of Augustine's more extreme views. Below are some quotes from Augustine's works themselves:
Ad Simplicianum 1, 2, 16: "Therefore all men are . . . one condemned mass [massa damnata] of sin, that owes a debt of punishment to the divine and supreme justice. Whether it [the debt] be exacted, or whether it be condoned, there is no injustice."
Enchiridion 27: ". . . the whole condemned mass of the human race lay in evils, or even rolled about in them, and was precipitated from evils into evils. . . ."
City of God 21, 12: "Hence there is a condemned mass of the whole human race . . . so that no one would be freed from this just and due punishment except by mercy and undue grace; and so the human race is divided [into two parts] so that in some it may be shown what merciful grace can do, in others, what just vengeance can do. . . . In it [punishment] there are many more than in [mercy] so that in this way there may be shown what is due to all."
Epistle 190. 3. 12: He said that reprobates are so much more numerous than the saved that "by an incomparable number they are more numerous than those whom He deigned to predestine as sons of the promise to the glory of His kingdom; so that by the very number of those rejected, it might he shown that the number, howsoever large, of the justly damned is of no importance with a just God. . . ." Which implies that God does not will all to be saved: hence Augustine's explicit denial, several times, of the words of 1 Tim 2:4. Hence too, as we said above, God does not really love anyone: He merely uses a few to show mercy.
Enchiridion 99: "For grace alone distinguishes the redeemed from the lost, whom a common cause from [their] beginning had joined into one mass of perdition. . . ."
On correction and grace 13, 42: "Those, then, who do not belong to that most certain and most happy number [of the predestined] are judged most justly according to their merits. For they either lie under the sin which they contracted originally by generation.
I speak thus of those who are predestined to the Kingdom of God, whose number is so certain that none may be added to or subtracted therefrom ... while those who do not belong to this most certain and blessed number are most righteously judged according to their deservings. For they lie under the sin which they have inherited by original generation and so depart hence with the inherited debt [On Rebuke and Grace, XIII, 39 940, 42 942].
It is not therefore a matter of man's willing, of his running, but of God's mercy (cf. Rom. 9:16), saying not of man's willing or running, but God's mercy means precisely that the entire process is credited to God, Who prepares the will and helps the will thus prepared [Ench., 32 248.].
Thus both the twins (Jacob and Esau) were born children of wrath, not on account of any works of their own, but because they were bound in the fetters of that original condemnation which came through Adam. But He Who said, I will have mercy on whom I will have mercy; loved Jacob of His undeserved grace, and hated Esau of His deserved judgment [The Enchiridion, Nicene Fathers, 1st Ser., Vol. 3, Ch. 98, Predestination to Eternal Life is Wholly of God's Free Grace].
God foreknew believers; but He chose them that they might be so, not because they were already so... He did not foresee that we ourselves would be holy and blameless, but He chose and predestined us that we might be so [On the Predestination of the Saints, XVII, 34 PL 44:985].
But it seems to men that all who appear good believers ought to receive perseverance to the end. But God has judged it to be better to mingle some who would not persevere with a certain number of His saints, so that those for whom security from temptation in this life is not desirable may not be secure. For that which the apostle says, checks many from mischievous elation: “Wherefore let him who seems to stand take heed lest he fall.†1 Cor. x. 12. But he who falls, falls by his own will, and he who stands, stands by God’s will. “For God is able to make him stand;†Rom. xiv. 4. therefore he is not able to make himself stand, but God [On the Gift of Perseverance, 19].
They Who Have Not Received Perseverance are Not Distinguished from the Mass of Those that are Lost [On Rebuke and Grace, XII, 36 938]
If you wish to be a catholic, do not venture to believe, to say, or to teach that they whom the Lord has predestinated for baptism can be snatched away from his predestination, or die before that has been accomplished in them which the Almighty has predestined [Book 3, Addressed to Vincentius Victor, Nicene, 1st Ser., Vol. V, Chapter 13 - His Seventh Error. In the Shape of a Letter Addressed to Presbyter Peter]
Moreover, no one would be saved if God had not brought aid to the infirmity of the human will, so that it might be unchangeably and invincibly motivated by divine grace... Even though the will of the elect may be weak and incapable of good, God prevents their defection [Book 3, Addressed to Vincentius Victor, Nicene, 1st Ser., Vol. V, Chapter 38 940].
only a few are saved by faith, a faith which they possess by virtue of their predestination to glory [On the Predestination of the Saints, XVII, 34 985].
They have been made vessels of wrath, and were born to the advantage of the saved... God knows what good may be made of them... Yet, He leads none of them to the salutary and spiritual repentance by which a man in Christ is reconciled to God [Contr Jul V, iv, 14 PL 44:792,793]
These are they who are predestinated and called according to the purpose, of whom not one perishes. And therefore none of them ends this life when he has changed from good to evil, because he is so ordained, and for that purpose given to Christ, that he may not perish, but may have eternal life [Treatise on Rebuke and Grace, Ch 21].
I think, too, that I have so discussed the subject that it is not so much myself as the inspired Scriptures which have spoken to you in the most vivid testimonies of truth; and if this divine record be looked into carefully, it reveals that God Himself converts the will of man from evil to good and that once it is converted, He directs him to good actions and eternal life; but also, that those who follow after the world are so at the disposal of God that He turns them wherever and whenever He wills  to bestow kindness on some and heap punishment on others, as He Himself judges rightly by a counsel most secret to Himself [On Grace and Free Will, 41]
