"Christ is born, glorify ye Him. Christ from heaven, go ye out to meet Him. Christ on earth; be ye exalted. Sing unto the Lord all the whole earth; and that I may join both in one word, Let the heavens rejoice, and let the earth be glad, for Him Who is of heaven and then of earth. Christ in the flesh, rejoice with trembling and with joy; with trembling because of your sins, with joy because of your hope. Christ of a Virgin; O ye Matrons live as Virgins, that ye may be Mothers of Christ. Who doth not worship Him That is from the beginning? Who doth not glorify Him That is the Last?"
"For God was manifested to man by birth. On the one hand Being, and eternally Being, of the Eternal Being, above cause and word, for there was no word before The Word; and on the other hand for our sakes also Becoming, that He Who gives us our being might also give us our Well-being, or rather might restore us by His Incarnation, when we had by wickedness fallen from wellbeing. The name Theophany is given to it in reference to the Manifestation, and that of Birthday in respect of His Birth. This is our present Festival; it is this which we are celebrating to-day, the Coming of God to Man, that we might go forth, or rather (for this is the more proper expression) that we might go back to God--that putting off the old man, we might put on the New; and that as we died in Adam, so we might live in Christ, being born with Christ and crucified with Him and buried with Him and rising with Him. For I must undergo the beautiful conversion, and as the painful succeeded the more blissful, so must the more blissful come out of the painful. For where sin abounded Grace did much more abound; and if a taste condemned us, how much more doth the Passion of Christ justify us? Therefore let us keep the Feast, not after the manner of a heathen festival, but after a godly sort; not after the way of the world, but in a fashion above the world; not as our own but as belonging to Him Who is ours, or rather as our Master's; not as of weakness, but as of healing; not as of creation, but of re-creation."
The above selecton is from St. Gregory the Theologian, Oration XXXVIII, "On the Theophany, or Birthday of Christ," NPNF.