from a letter by Pope St Leo the Great
Lowliness is assured by majesty, weakness by power, mortality by eternity. To pay the debt of our sinful state, a nature that was incapable of suffering was joined to one that could suffer. Thus, in keeping with the healing that we needed, one and the same mediator between God and men, the man Jesus Christ, was able to die in one nature, and unable to die in the other.
He who is true God was therefore born in the complete and perfect nature of a true man, whole in his own nature, whole in ours. By our nature we mean what the Creator had fashioned in us from the beginning, and took to himself in order to restore it.For in the Saviour there was no trace of what the deceiver introduced and man, being misled, allowed to enter. It does not follow that because he submitted to sharing in our human weakness he therefore shared in our sins.
He took the nature of a servant without stain of sin, enlarging our humanity without diminishing his divinity. He emptied himself; though invisible he made himself visible, though Creator and Lord of all things he chose to be one of us mortal men. Yet this was the condescension of compassion, not the loss of omnipotence. So he who in the nature of God had created man, became in the nature of a servant, man himself.
Thus the Son of God enters this lowly world. He comes down from the throne of heaven, yet does not separate himself from the Father’s glory. He is born in a new condition, by a new birth.