6 But our impiety, by the licence of this forbidden language, waxes apace with yet deeper faithlessness; asserting that since the Son is a creature it is bound to maintain that the Father also does not differ from a creature. For Christ, remaining in the form of God, took the form of a servant; and if He is a creature Who is in the form of God, God can never be separate from the creature, because there is a creature in the form of God. But to be in the form of God can only be understood to mean, remaining in the nature of God; whence also God is a creature, because there is a creature with His nature.
But He Who was in the form of God, did not grasp at being equal with God, because from equality with God, that is, from the form of God, He descended into the form of a servant. But He could not descend from God into man, except by emptying Himself, as God, of the form of God. But when He emptied Himself, He was not effaced, so as not to be; since then He would have become other in kind than He had been. For neither did He, Who emptied Himself within Himself, cease to be Himself; since the power of His might remains even in the power of emptying Himself; and the transition into the form of a servant does not mean the loss of the nature of God, since to have put off the form of God is nothing less than a mighty act of divine power.
Source: On the Trinity (New Advent)