18 Thus, when we read, Let Us make man after Our image and likeness, these two words Us and Our reveal that there is neither one isolated God, nor yet one God in two dissimilar Persons; and our confession must be framed in harmony with the second as well as with the first truth. For the words our image— not our images— prove that there is one nature possessed by Both. But an argument from words is an insufficient proof, unless its result be confirmed by the evidence of facts; and accordingly it is written, And God made man; after the image of God made He him. If the words He spoke, I ask, were the soliloquy of an isolated God, what meaning shall we assign to this last statement?
For in it I see a triple allusion, to the Maker, to the being made, and to the image. The being made is man; God made him, and made him in the image of God. If Genesis were speaking of an isolated God, it would certainly have been And made him after His own image. But since the book was foreshowing the Mystery of the Gospel, it spoke not of two Gods, but of God and God, for it speaks of man made through God in the image of God. Thus we find that God wrought man after an image and likeness common to Himself and to God; that the mention of an Agent forbids us to assume that He was isolated; and that the work, done after an image and likeness which was that of Both, proves that there is no difference in kind between the Godhead of the One and of the Other.
Source: On the Trinity (New Advent)