St. Gregory the Theologian, Oration 38, Section 3:
These things come later. Now is the feast of the Theophany, and so also of the Nativity; for it is called both, since two names are ascribes to one reality. For God appeared to human beings through birth. On the one hand he is and is eternally from the eternal Being, above cause and principle, for there was no principle higher than the Principle. On the other hand for us he later comes into being, that the one who has given us being might also grant us well-being; or rather that, as we fell from well being through evil, he might bring us back again to himself through incarnation. The name Theophany, since he has appeared, and Nativity, since he has been born.