John Chrysostom — on Phil 2:7 (Homily on Philippians 6)

“What shall we say against Arius, who asserts the Son is of a different substance? Tell me now, what means, "He took the form of a servant"? It means, He became man. Wherefore "being in the form of God," He was God. For one "form" and another "form" is named; if the one be true, the other is also. "The form of a servant" means, Man by nature, wherefore "the form of God" means, God by nature. I said that the "form of a servant" was a true form, and nothing less. Therefore "the form of God" also is perfect, and no less. Why says he not, "being made in the form of God," but "being in the form of God"? This is the same as the saying, "I am that I am." "Form" implies unchangeableness, so far as it is form. It is not possible that things of one substance should have the form of another, as no man has the form of an angel, neither has a beast the form of a man. How then should the Son?”