48 But neither is it necessary for the Only-begotten God that He should change. He is God, and that is the name of full and perfect divinity. For, as we said before, the meaning of the repeated glorifying, and the cause of the subjection is that God may be all in all: but it is a Mystery, not a necessity, that God is to be all in all. Christ abode in the form of God when He assumed the form of a servant, not being subjected to change, but emptying Himself; hiding within Himself, and remaining master of Himself though He was emptied. He constrained Himself even to the form and fashion of a man, lest the weakness of the assumed humility should not be able to endure the immeasurable power of His nature. His unbounded might contracted itself, until it could fulfil the duty of obedience even to the endurance of the body to which it was yoked. But since He was self-contained even when He emptied Himself, His authority suffered no diminution, for in the humiliation of the emptying He exercised within Himself the power of that authority which was emptied.
Source: On the Trinity (New Advent)