Since the image of God had been destroyed in us by the fall, we may judge from its restoration what it originally had been. Paul says that we are transformed into the image of God by the gospel. And, according to him, spiritual regeneration is nothing else than the restoration of the same image. (Colossians 3:10, and Ephesians 4:23.) That he made this image to consist in righteousness and true holiness, is by the figure synecdoche; for though this is the chief part, it is not the whole of God's image. Therefore by this word the perfection of our whole nature is designated, as it appeared when Adam was endued with a right judgment, had affections in harmony with reason, had all his senses sound and well-regulated, and truly excelled in everything good. Thus the chief seat of the Divine image was in his mind and heart, where it was eminent: yet was there no part of him in which some scintillations of it did not shine forth. For there was an attempering in the several parts of the soul, which corresponded with their various offices. In the mind perfect intelligence flourished and reigned, uprightness attended as its companion, and all the senses were prepared and moulded for due obedience to reason; and in the body there was a suitable correspondence with this internal order. But now, although some obscure lineaments of that image are found remaining in us; yet are they so vitiated and maimed, that they may truly be said to be destroyed. For besides the deformity which everywhere appears unsightly, this evil also is added, that no part is free from the infection of sin.
Later, in discussing Genesis 5:1, he says:
Moreover, Moses again repeats what he had before stated that Adam was formed according to the image of God, because the excellency and dignity of this favor could not be sufficiently celebrated. It was already a great thing, that the principal place among the creatures was given to man; but it is a nobility far more exalted, that he should bear resemblance to his Creator, as a son does to his father. It was not indeed possible for God to act more liberally towards man, than by impressing his own glory upon him, thus making him, as it were, a living image of the Divine wisdom and justice.
And on the important image of God passage in Genesis 9:6:
For the greater confirmation of the above doctrines God declares, that he is not thus solicitous respecting human life rashly, and for no purpose. Men are indeed unworthy of God's care, if respect be had only to themselves. but since they bear the image of God engraven on them, He deems himself violated in their person. Thus, although they have nothing of their own by which they obtain the favor of God, he looks upon his own gifts in them, and is thereby excited to love and to care for them. This doctrine, however is to be carefully observed that no one can be injurious to his brother without wounding God himself. Were this doctrine deeply fixed in our minds, we should be much more reluctant than we are to inflict injuries. Should any one object, that this divine image has been obliterated, the solution is easy; first, there yet exists some remnant of it, so that man is possessed of no small dignity; and, secondly, the Celestial Creator himself, however corrupted man may be, still keeps in view the end of his original creation; and according to his example, we ought to consider for what end he created men, and what excellence he has bestowed upon them above the rest of living beings.
The same basic point is repeated in the Commentary on Colossians:
And this is what he immediately adds, that we are renewed after the image of God. Now, the image of God resides in the whole of the soul, inasmuch as it is not the reason merely that is rectified, but also the will. Hence, too, we learn, on the one hand, what is the end of our regeneration, that is, that we may be made like God, and that his glory may shine forth in us; and, on the other hand, what is the image of God, of which mention is made by Moses in Genesis 9:6, the rectitude and integrity of the whole soul, so that man reflects, like a mirror, the wisdom, righteousness, and goodness of God. He speaks somewhat differently in the Epistle to the Ephesians, but the meaning is the same. See the passage -- Ephesians 4:24. Paul, at the same time, teaches, that there is nothing more excellent at which the Colossians can aspire, inasmuch as this is our highest perfection and blessedness to bear the image of God.
In the Commentary on Genesis, Calvin, interestingly enough, is very critical of Augustine's very influential meditations on the image of God in De Trinitate:
But Augustine, beyond all others, speculates with excessive refinement, for the purpose of fabricating a Trinity in man. For in laying hold of the three faculties of the soul enumerated by Aristotle, the intellect, the memory, and the will, he afterwards out of one Trinity derives many. If any reader, having leisure, wishes to enjoy such speculations, let him read the tenth and fourteenth books on the Trinity, also the eleventh book of the "City of God." I acknowledge, indeed, that there is something in man which refers to the Father and the Son, and the Spirit: and I have no difficulty in admitting the above distinction of the faculties of the soul: although the simpler division into two parts, which is more used in Scripture, is better adapted to the sound doctrine of piety; but a definition of the image of God ought to rest on a firmer basis than such subtleties.
Note, however, that this criticism falls short of an absolute rejection; his concern is with what he sees as "excessive refinement for the purpose of fabricating a Trinity in man" and too much tendency to rest the definition of the image of God on "subtleties". He accepts the basic Augustinian move, however, when he says that "there is something in man which refers to the Father and the Son and the Spirit," although it isn't clear from this how far he is willing to go.