God becomes man by Proclus of Constantinople

What, then, is the mystery celebrated in yesterday’s solemnity? The unexplainable mystery of the divinity and the humanity, a birth that leaves the Mother uncorrupt, an Incarnation that gives a form to the incorporeal Divinity, while it undergoes no passion, an extraordinary birth, a beginning for a generated One who has no beginning.
For origin belongs to his human nature, while his divinity has no origin . ..; the union of two natures and the birth of a single Son, the unconfused union of the Word with the flesh. He who yesterday was born according to the flesh is God because of what he received from the Father, but he is man because of what he took from me.
O tremendous and marvelous mystery! Who ever saw a king wearing the garments of a condemned man? What eye ever contained all of the sun’s light? When was a body ever united substantally and without change to a God, other than yesterday?

Proclus of Constantinople, Homily 3 on the Incarnation; PG 65, 703-8