11 We do not know Christ the God unless we know God the Begotten. But to be born God is to belong to the nature of God, for the name Begotten signifies indeed the manner of His origin, but does not make Him different in kind from the Begetter. And if so, the Begotten owes indeed to His Author the source of His being, but is not dispossessed of the nature of that Author, for the birth of God can arise but from one origin, and have but one nature. If its origin is not from God, it is not a birth; if it is anything but a birth, Christ is not God. But He is God of God, and therefore God the Father stands to God the Son as God of His birth and Father of His nature, for the birth of God is from God, and in the specific nature of God.
Source: On the Trinity (New Advent)