St Cyril of Alexandria, On the Incarnation
God the Word full by nature and in every way Perfect, and distributing out of His own Fullness His own goods to the creature, we say was emptied: in no wise wronged in His own Proper Nature, nor changed so as to become otherwise, nor made in ought inferior, for inconvertible and unchangeable is Himself also even as He Who begat Him, and never may He be capable of passion. But when He was made Flesh, i.e. Man, He made (as He said, I will pour forth of My Spirit upon all flesh) the poverty of human nature His own; first, in that He was once made man, albeit He remained God; next in that He took the form of a servant, Who is in His own Nature free, as Son, and while He is Himself the Lord of glory He is said to receive glory: Himself Life, He is said to be quickened: and receives power over all, Himself King of all and with God, and Ho was obedient to the Father, suffered the Cross and so on. But these things befit the measure of the human nature, yet He makes them His own with flesh and fulfils the economy, remaining what He was.
St Gregory the theologian, Orations
The very Son of God, older than the ages, the invisible, the incomprehensible, the incorporeal, the beginning of beginning, the light of light, the fountain of life and immortality, the image of the archetype, the immovable seal, the perfect likeness, the definition and word of the Father: he it is who comes to his own image and takes our nature for the good of our nature, and unites himself to an intelligent soul for the good of my soul, to purify like by like. He takes to himself all that is human, except for sin. He was conceived by the Virgin Mary, who had been first prepared in soul and body by the Spirit; his coming to birth had to be treated with honor, virginity had to receive new honor. He comes forth as God, in the human nature he has taken, one being, made of two contrary elements, flesh and spirit. Spirit gave divinity, flesh received it.
He who makes rich is made poor; he takes on the poverty of my flesh, that I may gain the riches of his divinity. He who is full is made empty; he is emptied for a brief space of his glory, that I may share in his fullness. What is this wealth of goodness? What is this mystery that surrounds me? I received the likeness of God, but failed to keep it. He takes on my flesh, to bring salvation to the image, immortality to the flesh. He enters into a second union with us, a union far more wonderful than the first.
The Fast of the Incarnation
St Athanasius, Patriachal Letters
Behold my brothers, how much a fast can do, and in what manner the law commands us to fast. It is required that not only with the body should we fast, but with the soul. Now the soul is humbled when it does not follow wicked opinions, but feeds on becoming virtues. For virtues and vices are the food of the soul and it can eat either of these two meats, and incline to either of the two, according to its own will. If it is bent toward virtue, it will be nourished by virtues, by righteousness, by temperance, by meekness and by fortitude. Such was the case with our Lord, Who said, 'My food is to do the will of My Father Who is in heaven' (Jn. 4:34). But if it is not thus with the soul, and it inclines downwards, it is then nourished by nothing but sin.