8 To assure us still more fully of the true Godhead manifested in the nature and work of the Son, He, Who expressed His meaning in the words I have cited, shows that His thought was suggested by the true Divinity of Him to Whom He said, After Our own image and likeness. How is He falsely called God, to Whom the true God says, After Our own image and likeness? Our is inconsistent with isolation, and with difference either in purpose or in nature.
Man is created, taking the words in their strict sense, in Their common image. Now there can be nothing common to the true and to the false. God, the Speaker, is speaking to God; man is being created in the image of Father and of Son. The Two are One in name and One in nature. It is only one image after which man is made. The time has not yet come for me to discuss this matter; hereafter I will explain what is this image of God the Father and of God the Son into which man was created.
For the present we will stick to the question, was, or was not, He true God, to Whom the true God said, Let Us make man after Our own image and likeness? Separate, if you can, the true from the false elements in this image common to Both; in your heretical madness divide the indivisible. For They Two are One, of Whose one image and likeness man is the one copy.
Source: On the Trinity (New Advent)