He cites Scripture at great length, and especially the Pauline Epistles, which establish for him salvation received by grace alone - the initiative is entirely God’s, who elects whom he wills, through faith apart from works performed in advance of reception, and faith itself the gift of God. That is to say, his anti-Pelagian writings in particular are replete with Pauline-inspired discussions of this kind, which do not call upon him to clarify repeatedly that justifico basically means "to make righteous", or to show his readers how he understands the gift of justification - of being jusitificati - in relation to this normal meaning. 
I believe that it is in such general terms as these that Calvin rather guardedly appropriates Augustine on justification. Augustine sees clearly that justification (however exactly understood) is by grace alone. This is repeatedly expressed in the Anti-Pelagian writings which were such a rich resource for the Reformers in establishing their views of the 'servitude' of the human will and the freeness and power of divine grace.
Accurate as this may be as a view of Augustine's position, Calvin does not quite see him this way, for there is not much evidence that he identifies Augustine as even toying with the idea of justification by faith in a declarative sense, even though, as we have seen, Augustine may have done so, perhaps committing himself to that view (without realizing it) in what he writes. After all, a person might not be as aware as others are of the logical implications of views that he holds.
We can reconstruct Calvin's view of Augustine on justification by considering two lines of evidence. First by noting a striking fact, that throughout his discussion of justification Calvin cites Augustine voluntarily (that is, he is not forced into a citation through the pressure of controversy) and almost wholly with approval. The second line of evidence is the reasons that he provides where he thinks that Augustine is defective.
Here are some of the places where Calvin records his approval of Augustine.
And lest you suppose that there is anything novel in what I say, Augustine has also taught us so to act [viz. To pay no regard to our works for justification]. "Christ”, says he, "will reign forever among his servants. This God has promised, God has spoken; if this is not enough, God has sworn. Therefore, as the promise stands firm, not in respect of our merits, but in respect of his mercy, no one ought to tremble in announcing that of which he cannot doubt”.
Besides, if it is true, as John says, that there is no life without the Son of God (I John. 5.12), those who have no part in Christ, whoever they be, whatever they do or devise, are hastening on, during their whole career, to destruction and the judgment of eternal death. For this reason, Augustine says, ‘Our religion distinguishes the righteous from the wicked, by the law, not of works, but of faith, without which works which seem good are converted into sins’. 
The same thing is briefly but elegantly expressed by Augustine when he says, ‘I do not say to the Lord, Despise not the works of my hands; I have sought the Lord with my hands, and have not been deceived. But I commend not the works of my hands, for I fear that when thou examinest them thou wilt find more faults than merits. This only I say, this ask, this desire, Despise not the works of thy hands. See in me thy work, not mine. If thou sees mine, thou condemnest; if thou sees thine own, thou crownest . Whatever good works I have are of thee’. 
It is in this fairly regular way that Augustine (and to a lesser extent Bernard) are cited to in order to emphasise sola gratia. Sometimes the citations are for a positive purpose, sometimes negatively. Positively, that salvation is due only to the merits of Christ, and negatively, our own supposed 'merits' count for nothing as regards forgiveness and righteousness, no ground of boasting, because only the merits of Christ count, and God working his graces in us.
With this line of evidence Calvin sometimes contrasts Peter Lombard (whom he calls the 'Pythagoras' of the later Sophists) who, though he had Augustine 'so often in his mouth' failed in his blindness to see that Augustine ascribed to man not the least particle of praise because of good works; and also he contrasts him with the Schoolmen who teach that works have their value from divine 'accepting grace'. And he is scathing about the 'schools of the Sorbonne' to which he gives separate attention in his Antidote to the Articles Agreed Upon by the Faculty of Sacred Theology of Paris. 
Despite this widespread positive use of Augustine, there are two issues on which Calvin faults him. The first has to do with his use of the term 'merit'. Calvin includes Augustine in a general condemnation of the introduction of the word into discussions of human charactr and action. Nonetheless, Calvin says, Augustine used it circumspectly.
I admit it was used by ancient ecclesiastical writers, and I wish they had not by the abuse of one term furnished posterity with matter of heresy, although in some passages they themselves show that they had no wish to injure the truth. For Augustine says ‘Let human merits, which perished by Adam, here be silent, and let the grace of God reign by Jesus Christ’…. You see how he denies man the power of acting aright, and thus lays merit prostrate. 
The important point For Calvin here is obvious. Although Augustine and Bernard use the term ‘merit’ they do not reckon that the person who enjoys grace has himself merited it. The worth of the act is not due to an action of the person who performs it, but solely to divine grace.
Secondly, and more centrally, Calvin notes that for Augustine justificare connotes subjective renewal. Reviewing the way in which the biblical idea of justification had degenerated in the church, Calvin says, in the first instance about Lombard,
You see here that the chief office of divine grace in our justification he considers to be its directing us to good works by the agency of the Holy Spirit. He intended, no doubt, to follow the opinion of Augustine, but he follows it at a distance, and even wanders far from a true imitation of him, both obscuring what was clearly stated by Augustine, and making what in him was less pure more corrupt. The Schools have always gone from worse to worse, until at length, in their downward path, they have degenerated into a kind of Pelagianism. Even the sentiment of Augustine, or at least his mode of expressing it, cannot be entirely approved of. For although he is admirable in stripping man of all merit of righteousness, and transferring the whole praise of it to God, he classes the grace by which we are regenerated to newness of life under the head of sanctification. Scripture, when it treats of justification by faith, leads us in a very different direction. Turning away our view from our own works, it bids us look only to the mercy of God, and the perfection of Christ.
That is, in Calvin’s view Augustine subsumes ‘grace’, that is, the grace of justification, under sanctification, subjective renewal. This is his account of Augustine’s doctrine of grace using Reformation conceptuality. Not that it is a meritorious consequence of renewal, for renewal is also the fruit of grace, but in Calvin’s view Augustine holds that a person is justified as he is being renewed, and inbeing renewed. Apart from anything else, Calvin wishes to make space for the Pauline assertion that God justifies the ungodly. On Augustine’s view of justification, God justifies the ungodly, but Calvin believes that he means something different (from the Reformers) by ‘justification’.
It is not unknown to me, that Augustine gives a different explanation; for he thinks that the righteousness of God is the grace of regeneration; and this grace he allows to be free, because God renews us, when unworthy, by his Spirit; and from this he excludes the works of the law, that is, those works, by which men of themselves endeavour, without renovation, to render God indebted to them.... But that the Apostle includes all works without exception, even those which the Lord produces in his own people, is evident from the context. 
He makes a similar point, though without mentioning Augustine, as follows
There is no controversy between us and the sounder Schoolmen as to the beginning of justification. They admit that the sinner, freely delivered from condemnation, obtains justification, and that by forgiveness of sins; but under the term justification they comprehend the renovation by which the Spirit forms us anew to the obedience of the Law; and in describing the righteousness of the regenerate man, maintain that being once reconciled to God by means of Christ, he is afterward deemed righteous by his good works, and is accepted in consideration of them.
There is ambivalence here, a certain awkwardness. On the one hand, we must not entirely approve of Augustine's thinking, ‘or at least his mode of expressing it’. This suggests a mere verbal disagreement. On the other hand, the Bible's way of thinking is 'leads us in a very different direction'. What is it in Augustine’s way of expressing what he thinks that we may not approve of? It is not merely that Augustine uses the term ‘merit’, because that term can be given a good sense, even though (in Calvin’s eyes) it came in the medieval church to have a very bad sense. Augustine can hardly be blamed for that. Rather it is that he muffles the vital point that justification and sanctification are not only inseparable but also distinct. For in the Augustinian way of thinking, while there is agreement that justification involves freedom from condemnation through forgiveness and the provision of righteousness, and that faith is active in it, subjective renewal is included in it. It is this merging of the two that, in Calvin's view, eventually led to appealing to good works as meritorious, and to the idea of supererogation on which the scandalous medieval abuses relied. Justification and sanctification are inseparable and distinct.
 Wright 59-60
 Inst. III.13.4. The quotation is from Augustine’s narration on Psalm 88, tract. 50.
 Inst. III.14.4. The Augustine quotation is from Against Two letters of the Pelagians, 3.5
 Inst. III.14.20. The quotation is from Augustine on Psalm 137. See also Inst. III.11.22, III.14.3, III.18.5, III.18.7.
 Inst. III.15.7 See also Calvin's further reference to 'accepting grace', Inst. III.14.12.
 Inst. III.15.7, III.18.9.
 Selected Works of John Calvin, Vol. I.
 Inst. III.15.2. The Augustine quotation is from The Predestination of the Saints. In the same section Calvin also makes a similar reference to Bernard.
 Inst. III.11.15-6.
 Comm. Rom. 3.22
 Inst. III.14.11