God is in flesh. He is not active at intervals as he was among the prophets. Instead he possesses a humanity connatural and united to himself, and restores all humanity to himself through flesh the same as ours in kind.
So then, one might say, ‘How did the splendour come to all by means of the one? How can Divinity come to be in flesh?’ As fire comes to be in iron: not by a change of place, but by a sharing of itself. For the fire does not go out of itself and into the iron; rather, while remaining in its place, it shares its own power with the iron. It is in no way diminished when it shares itself, and the whole of it fills whatever shares in it. So it is in this way too that God the Word did not move out of Himself when He dwelt among us. Nor did he undergo a change when the Word became flesh. Heaven was not deprived of what it contained, and earth received the heavenly one within its own embraces. Do not imagine hat the Divinity was altered when it was transferred into flesh. For the immortal is immutable.
So then, one might ask, ‘How was God the Word not filled with bodily weakness?’ We reply: as the fire does not share in the distinguishing marks of the iron. iron is black and cold, but nonetheless when turned in the fire it takes on the outward form of fire. The iron glows, yet the fire is not blackened. The iron is set ablaze, yet it does not col the flame. So too it is with the human flesh of the Lord: it shares in the divinity, yet it does not impart its own weakness to the divinity. Can it be that you did not grant to the divinity an activity on par with that of mortal fire? Did you imagine passibility in in the impassible one on the basis of human weakness? Are you puzzled how the easily corruptible nature can have incorruptibility through its communion with God? Realize that it’s a mystery. God is in flesh that he may kill the death that lurks therein. For as the harm caused by poisonous drugs can be overcome by antidotes when they are assimilated by the body, and as the darkness residing in a house is dissolved by the introduction of light, so too the death that dominates in human nature is obliterated by the presence of divinity. And as ice in water, for as long as it is night and dark, is stronger than the liquid that contains it, but the warming sun melts the ice by its ray, so too death rules until the advent of Christ, but when the saving grace of God appears (Titus 2.11) and the sun of righteousness rises (Mal 4.2), death is swallowed up in victory (1 Cor 15.54), unable to bear the visitation of true life.
O the depth of the goodness of God and his love for humanity! In response to his superabundant love for humanity we rebel against being his servants. We seek to know the reason by God is among humans, though we should be adoring His goodness.” St. Basil the Great, Homily on The Holy Birth of Christ, Chapter 2. Translation from the beautiful little collection On Fasting and Feasts