Why We Do Not in the Trinity Speak of One Person, and Three Essences. What He Ought to Believe Concerning the Trinity Who Does Not Receive What is Said Above. Man is Both After the Image, and is the Image of God
12 And until he be purged from this uncleanness, let him believe in the Father, Son, and Holy Spirit, one God, alone, great, omnipotent, good, just, merciful, Creator of all things visible and invisible, and whatsoever can be worthily and truly said of Him in proportion to human capacity. And when he is told that the Father only is God, let him not separate from Him the Son or the Holy Spirit; for together with Him He is the only God, together with whom also He is one God; because, when we are told that the Son also is the only God, we must needs take it without any separation of the Father or the Holy Spirit.
And let him so say one essence, as not to think one to be either greater or better than, or in any respect differing from, another. Yet not that the Father Himself is both Son and Holy Spirit, or whatever else each is singly called in relation to either of the others; as Word, which is not said except of the Son, or Gift, which is not said except of the Holy Spirit. And on this account also they admit the plural number, as it is written in the Gospel, “I and my Father are one.” He has both said “one,” and “we are one,” according to essence, because they are the same God; “we are,” according to relation, because the one is Father, the other is Son.
Sometimes also the unity of the essence is left unexpressed, and the relatives alone are mentioned in the plural number: “My Father and I will come unto him, and make our abode with him.” We will come, and we will make our abode, is the plural number, since it was said before, “I and my Father,” that is, the Son and the Father, which terms are used relatively to one another. Sometimes the meaning is altogether latent, as in Genesis: “Let us make man after our image and likeness.” Both let us make and our is said in the plural, and ought not to be received except as of relatives.
For it was not that gods might make, or make after the image and likeness of gods; but that the Father, and Son, and Holy Spirit might make after the image of the Father, and Son, and Holy Spirit, that man might subsist as the image of God. And God is the Trinity. But because that image of God was not made altogether equal to Him, as being not born of Him, but created by Him; in order to signify this, he is in such way the image as that he is “after the image,” that is, he is not made equal by parity, but approaches to Him by a sort of likeness.
For approach to God is not by intervals of place, but by likeness, and withdrawal from Him is by unlikeness. For there are some who draw this distinction, that they will have the Son to be the image, but man not to be the image, but “after the image.” But the apostle refutes them, saying, “For a man indeed ought not to cover his head, forasmuch as he is the image and glory of God.” He did not say after the image, but the image. And this image, since it is elsewhere spoken of as after the image, is not as if it were said relatively to the Son, who is the image equal to the Father; otherwise he would not say after our image.
For how our, when the Son is the image of the Father alone? But man is said to be “after the image,” on account, as we have said, of the inequality of the likeness; and therefore after our image, that man might be the image of the Trinity; not equal to the Trinity as the Son is equal to the Father, but approaching to it, as has been said, by a certain likeness; just as nearness may in a sense be signified in things distant from each other, not in respect of place, but of a sort of imitation.
For it is also said, “Be transformed by the renewing of your mind;” to whom he likewise says, “Be therefore imitators of God as dear children.” For it is said to the new man, “which is renewed to the knowledge of God, after the image of Him that created him.” Or if we choose to admit the plural number, in order to meet the needs of argument, even putting aside relative terms, that so we may answer in one term when it is asked what three, and say three substances or three persons; then let no one think of any bulk or interval, or of any distance of howsoever little unlikeness, so that in the Trinity any should be understood to be even a little less than another, in whatsoever way one thing can be less than another: in order that there may be neither a confusion of persons, nor such a distinction as that there should be any inequality.
And if this cannot be grasped by the understanding, let it be held by faith, until He shall dawn in the heart who says by the prophet, “If you will not believe, surely you shall not understand.”
Source: On the Holy Trinity (New Advent)