De Synodis 21

I. "If any one hearing that the Son is the image of the invisible God, says that the image of God is the same as the invisible God, as though refusing to confess that He is truly Son: let him be anathema." 13. Hereby is excluded the assertion of those who wish to represent the relationship of Father and Son as a matter of names, inasmuch as every image is similar in species to that of which it is an image. For no one is himself his own image, but it is necessary that the image should demonstrate him of whom it is an image. So an image is the figured and indistinguishable likeness of one thing equated with another. Therefore the Father is, and the Son is, because the Son is the image of the Father: and he who is an image, if he is to be truly an image, must have in himself his original's species, nature and essence in virtue of the fact that he is an image.