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.
Again the darkness is past; again Light is made; again Egypt is punished with darkness; again Israel is enlightened by a pillar (Exodus 14.20). The people that sat in the darkness of ignorance, let it see the Great Light of full knowledge (Isaiah 9.6). Old things are passed away, behold all things are become new (1 Cor 5.17). The letter gives way, the Spirit comes to the front. The shadows flee away, the Truth comes in upon them. Melchisedec is concluded. He that was without Mother becomes without Father (without Mother of His former state, without Father of His second)….
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 (Eph 4.22,24), 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 (1 Cor 15.22), being born with Christ and crucified with Him and buried with Him and rising with Him (Col 2.11). 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 (Rom 5.20); 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.
St. Gregory the Theologian, Oration 38, Tracts 1-4, The Oration on The Theophany or The Birth Day of Christ as in The Nicene and Post-Nicene Fathers Collection