Ambrose of Milan

It is one thing to be named Son according to the divine substance; it is another thing to be so called according to the adoption of human flesh. For, according to the divine generation, the Son is equal to God and Father, and, according to the adoption of a body, he is a servant to God the Father. "For," it says, "he took upon him the form of a servant." The Son is, however, one and the same.… According to his glory, he is Lord to the holy patriarch David but David's son in the line of actual descent, abandoning nothing of his own but acquiring for himself the rights that go with the adoption into our race. Not only does he undergo service in the character of man by reason of his descent from David, but also by reason of his name, as it is written: "I have found David my servant";4 and elsewhere: "Behold, I will send to you my Servant, the Orient is his name." And the Son himself says, "Thus says the Lord, that formed me from the womb to be his servant and said to me: It is a great thing for you to be called my servant. Behold, I have set you up for a witness to my people and a light to the Gentiles, that you may be for salvation to the ends of the earth." To whom is this said, if not to Christ? Who, being in the form of God, emptied himself and took on him the form of a servant. But what can be in the form of God, except that which exists in the fullness of the godhead?