He who is true God is also true man. There is no falsehood in this unity as long as the lowliness of man and the pre-eminence of God coexist in mutual relationship.
As God does not change by his condescension, so man is not swallowed up by being exalted. Each nature exercises its own activity, in communion with the other. The Word does what is proper to the Word, the flesh fulfils what is proper to the flesh.
One nature is resplendent with miracles, the other falls victim to injuries. As the Word does not lose equality with the Father’s glory, so the flesh does not leave behind the nature of our race.
One and the same person – this must be said over and over again – is truly the Son of God and truly the son of man. He is God in virtue of the fact that in the beginning was the Word, and the Word was with God, and the Word was God. He is man in virtue of the fact that the Word was made flesh, and dwelt among us.
From a letter by Saint Leo the Great, pope