This grace is habitual because it endures and persists until we forfeit it. Man can lose this sanctifying grace by freely and knowingly committing a sin involving grave matter (referred to as being subjectively guilty of a mortal sin). [cf. CCC 1856, 1859, 2000] Augustine on Final Preservation:
“But of such as these [the Elect] none perishes, because of all that the Father has given Him, He will lose none. John 6:39 Whoever, therefore, is of these does not perish at all; nor was any who perishes ever of these. For which reason it is said, They went out from among us, but they were not of us; for if they had been of us, they would certainly have continued with us. 1 John 2:19”. (Augustine, Treatise on the Predestination of the Saints)
“I assert, therefore, that the perseverance by which we persevere in Christ even to the end is the gift of God; and I call that the end by which is finished that life wherein alone there is peril of falling.” (Augustine, On the Perseverance of the Saints)
"Will any one dare to say that this perseverance is not the gift of God, and that so great a possession as this is ours in such wise that if any one have it the apostle could not say to him, 'For what hast thou which thou hast not received?'[ 2] since he has this in such a manner as that he has not received it?" To this, indeed, we are not able to deny, that perseverance in good, progressing even to the end, is also a great gift of God; and that it exists not save it come from Him of whom it is written, "Every best gift and every perfect gift is from above, coming down from the Father of lights." (Augustine, Treatise on Rebuke and Grace, Ch. 10)
"... the human will does not obtain grace by freedom, but obtains freedom by grace; when the feeling of delight has been imparted through. the same grace, the human will is formed to endure; it is strengthened with unconquerable fortitude; controlled by grace, it never will perish, but, if grace forsake it, it will straightway fall; by the Lord's free mercy it is converted to good, and once converted it perseveres in good; the direction of the human will toward good, and after direction its continuation in good, depend solely upon God's will, not upon any merit of man. Thus there is left to man such free will, if we please so to call it, as he elsewhere describes: that except through grace the will can neither be converted to God nor abide in God; and whatever it can do it is able to do only through grace. "(Augustine, Aurelius. Augustine's Writings on Grace and Free WIll (Kindle Locations 45-46). Monergism Books. Kindle Edition.)
Justification presupposes man’s free act of will to accept and cooperate with this grace. God does not force this action or state on man. Human freedom is a secondary, but, essential element.
Augustine teaches that salvation is by "man's free will," in the sense that God illuminates the heart, making it alive again, and with love irresistibly draws the sinner to salvation. In other words, God draws us willingly, but it is impossible for a blind man to be given sight and to not see the face of His God. IOW, this is not something that man can resist, nor can it be properly said to be "cooperation" in the Roman Catholic sense, that they earned it. Rather, it is by the will of God entirely, without the "willing" or "running" of man: More from Augustine on this topic here:
“And further, should any one be inclined to boast, not indeed of his works, but of the freedom of his will, as if the first merit belonged to him, this very liberty of good action being given to him as a reward he had earned, let him listen to this same preacher of grace, when he says: “For it is God which works in you, both to will and to do of His own good pleasure;” (Php 2:13) and in another place: “So, then, it is not of him that wills, nor of him that runs, but of God that shows mercy.” (Rom 9:16) Now as, undoubtedly, if a man is of the age to use his reason, he cannot believe, hope, love, unless he will to do so, nor obtain the prize of the high calling of God unless he voluntarily run for it; in what sense is it not of him that wills, nor of him that runs, but of God that shows mercy, except that, as it is written, “the preparation of the heart is from the Lord?” Otherwise, if it is said, “It is not of him that wills, nor of him that runs, but of God that shows mercy, because it is of both,” that is, both of the will of man and of the mercy of God, so that we are to understand the saying, “It is not of him that wills, nor of him that runs, but of God that shows mercy,” as if it meant the will of man alone is not sufficient, if the mercy of God go not with it—then it will follow that the mercy of God alone is not sufficient, if the will of man go not with it; and therefore, if we may rightly say, it is not of man that wills, but of God that shows mercy, because the will of man by itself is not enough, why may we not also rightly put it in the converse way: “It is not of God that shows mercy, but of man that wills,” because the mercy of God by itself does not suffice? Surely, if no Christian will dare to say this, “It is not of God that shows mercy, but of man that wills,” lest he should openly contradict the apostle, it follows that the true interpretation of the saying, “It is not of him that wills, nor of him that runs, but of God that shows mercy,” is that the whole work belongs to God, who both makes the will of man righteous, and thus prepares it for assistance, and assists it when it is prepared.” (Augustine, The Enchiridion on Faith, Hope and Love, Ch. 32)
“And, moreover, who will be so foolish and blasphemous as to say that God cannot change the evil wills of men, whichever, whenever, and wheresoever He chooses, and direct them to what is good? But when He does this He does it of mercy; when He does it not, it is of justice that He does it not for “He has mercy on whom He will have mercy, and whom He will He hardens.” And when the apostle said this, he was illustrating the grace of God, in connection with which he had just spoken of the twins in the womb of Rebecca, who “being not yet born, neither having done any good or evil that the purpose of God according to election might stand, not of works, but of Him that calls, it was said unto her, The elder shall serve the younger.” And in reference to this matter he quotes another prophetic testimony: “Jacob have I loved, but Esau have I hated.” But perceiving how what he had said might affect those who could not penetrate by their understanding the depth of this grace: “What shall we say then?” he says: “Is there unrighteousness with God? God forbid.” For it seems unjust that, in the absence of any merit or demerit, from good or evil works, God should love the one and hate the other. Now, if the apostle had wished us to understand that there were future good works of the one, and evil works of the other, which of course God foreknew, he would never have said, not of works, but, of future works, and in that way would have solved the difficulty, or rather there would then have been no difficulty to solve. As it is, however, after answering, God forbid; that is, God forbid that there should be unrighteousness with God; he goes on to prove that there is no unrighteousness in God’s doing this, and says: “For He says to Moses, I will have mercy on whom I will have mercy, and I will have compassion on whom I will have compassion.” “ (Augustine, The Enchiridion on Faith, Hope and Love, Chapter 98. Predestination to Eternal Life is Wholly of God’s Free Grace.)
"We know that God's grace is not given to all men . To those to whom it is given it is given neither according to the merits of works, nor according to the merits of the will, but by free grace. To those to whom it is not given we know that it is because of God's righteous judgment that it is not given." (Augustine, Treatise On Rebuke and Grace)
“But that world which God is in Christ reconciling unto Himself, which is saved by Christ, and has all its sins freely pardoned by Christ, has been chosen out of the world that is hostile, condemned, and defiled. For out of that mass, which has all perished in Adam, are formed the vessels of mercy, whereof that world of reconciliation is composed, that is hated by the world which belongeth to the vessels of wrath that are formed out of the same mass and fitted to destruction. Finally, after saying, “If ye were of the world, the world would love its own,” He immediately added, “But because ye are not of the world, but I have chosen you out of the world, therefore the world hateth you.” And so these men were themselves also of that world, and, that they might no longer be of it, were chosen out of it, through no merit of their own, for no good works of theirs had preceded; and not by nature, which through free-will had become totally corrupted at its source: but gratuitously, that is, of actual grace. For He who chose the world out of the world, effected for Himself, instead of finding, what He should choose: for “there is a remnant saved according to the election of grace. And if by grace,” he adds, “then is it no more of works: otherwise grace is no more grace.”” (Tractates on the Gospel of John, 15:17-19)
