Objection 1. It would seem that the image of the Divine Trinity is in the soul not only by comparison with God as its object. For the image of the Divine Trinity is to be found in the soul, as shown above (Article 7), according as the word in us proceeds from the speaker; and love from both. But this is to be found in us as regards any object. Therefore the image of the Divine Trinity is in our mind as regards any object.
Objection 2. Further, Augustine says (De Trin. xii, 4) that "when we seek trinity in the soul, we seek it in the whole of the soul, without separating the process of reasoning in temporal matters from the consideration of things eternal." Therefore the image of the Trinity is to be found in the soul, even as regards temporal objects.
Objection 3. Further, it is by grace that we can know and love God. If, therefore, the image of the Trinity is found in the soul by reason of the memory, understanding, and will or love of God, this image is not in man by nature but by grace, and thus is not common to all.
Objection 4. Further, the saints in heaven are most perfectly conformed to the image of God by the beatific vision; wherefore it is written (2 Corinthians 3:18): "We... are transformed into the same image from glory to glory." But temporal things are known by the beatific vision. Therefore the image of God exists in us even according to temporal things.
On the contrary, Augustine says (De Trin. xiv, 12): "The image of God exists in the mind, not because it has a remembrance of itself, loves itself, and understands itself; but because it can also remember, understand, and love God by Whom it was made." Much less, therefore, is the image of God in the soul, in respect of other objects.
I answer that, As above explained (2,7), image means a likeness which in some degree, however small, attains to a representation of the species. Wherefore we need to seek in the image of the Divine Trinity in the soul some kind of representation of species of the Divine Persons, so far as this is possible to a creature. Now the Divine Persons, as above stated (6,7), are distinguished from each other according to the procession of the word from the speaker, and the procession of love from both. Moreover the Word of God is born of God by the knowledge of Himself; and Love proceeds from God according as He loves Himself. But it is clear that diversity of objects diversifies the species of word and love; for in the human mind the species of a stone is specifically different from that of a horse, which also the love regarding each of them is specifically different. Hence we refer the Divine image in man to the verbal concept born of the knowledge of God, and to the love derived therefrom. Thus the image of God is found in the soul according as the soul turns to God, or possesses a nature that enables it to turn to God. Now the mind may turn towards an object in two ways: directly and immediately, or indirectly and mediately; as, for instance, when anyone sees a man reflected in a looking-glass he may be said to be turned towards that man. So Augustine says (De Trin. xiv, 8), the "the mind remembers itself, understands itself, and loves itself. If we perceive this, we perceive a trinity, not, indeed, God, but, nevertheless, rightly called the image of God." But this is due to the fact, not that the mind reflects on itself absolutely, but that thereby it can furthermore turn to God, as appears from the authority quoted above (Arg. On the contrary).
Reply to Objection 1. For the notion of an image it is not enough that something proceed from another, but it is also necessary to observe what proceeds and whence it proceeds; namely, that what is Word of God proceeds from knowledge of God.
Reply to Objection 2. In all the soul we may see a kind of trinity, not, however, as though besides the action of temporal things and the contemplation of eternal things, "any third thing should be required to make up the trinity," as he adds in the same passage. But in that part of the reason which is concerned with temporal things, "although a trinity may be found; yet the image of God is not to be seen there," as he says farther on; forasmuch as this knowledge of temporal things is adventitious to the soul. Moreover even the habits whereby temporal things are known are not always present; but sometimes they are actually present, and sometimes present only in memory even after they begin to exist in the soul. Such is clearly the case with faith, which comes to us temporally for this present life; while in the future life faith will no longer exist, but only the remembrance of faith.
Reply to Objection 3. The meritorious knowledge and love of God can be in us only by grace. Yet there is a certain natural knowledge and love as seen above (12, 12; 56, 3; 60, 5). This, too, is natural that the mind, in order to understand God, can make use of reason, in which sense we have already said that the image of God abides ever in the soul; "whether this image of God be so obsolete," as it were clouded, "as almost to amount to nothing," as in those who have not the use of reason; "or obscured and disfigured," as in sinners; or "clear and beautiful," as in the just; as Augustine says (De Trin. xiv, 6).
Reply to Objection 4. By the vision of glory temporal things will be seen in God Himself; and such a vision of things temporal will belong to the image of God. This is what Augustine means (De Trin. xiv, 6), when he says that "in that nature to which the mind will blissfully adhere, whatever it sees it will see as unchangeable"; for in the Uncreated Word are the types of all creatures.
Source: Summa Theologica (New Advent)