From The Orthodox Faith, III, 1:
For by the good pleasure of our God and Father, the Only-begotten Son and Word of God and God, Who is in the bosom of the God and Father John 1:18, of like essence with the Father and the Holy Spirit, Who was before the ages, Who is without beginning and was in the beginning, Who is in the presence of the God and Father, and is God and made in the form of God (Philippians 2:6), bent the heavens and descended to earth: that is to say, He humbled without humiliation His lofty station which yet could not be humbled, and condescends to His servants , with a condescension ineffable and incomprehensible: (for that is what the descent signifies). And God being perfect becomes perfect man, and brings to perfection the newest of all new things (Ecclesiastes 1:10), the only new thing under the Sun, through which the boundless might of God is manifested. For what greater thing is there, than that God should become Man? And the Word became flesh without being changed, of the Holy Spirit, and Mary the holy and ever-virgin one, the mother of God. And He acts as mediator between God and man, He the only lover of man conceived in the Virgin’s chaste womb without will or desire, or any connection with man or pleasurable generation, but through the Holy Spirit and the first offspring of Adam. And He becomes obedient to the Father Who is like us, and finds a remedy for our disobedience in what He had assumed from us, and became a pattern of obedience to us without which it is not possible to obtain salvation.