From the first Christmas Day sermon of Pope St. Leo the Great:
Dearly beloved brethren, Unto us is born this day a Saviour, Luke ii. 11. Let us rejoice. It would be unlawful to be sad to-day, for today is Life's Birthday; the Birthday of that Life, Which, for us dying creatures, taketh away the sting of death, and bringeth the bright promise of the eternal gladness hereafter. It would be unlawful for any man to refuse to partake in our rejoicing. All men have an equal share in the great cause of our joy, for, since our Lord, Who is the destroyer of sin and of death, findeth that all are bound under the condemnation, He is come to make all free. Rejoice, O thou that art holy, thou drawest nearer to thy crown! Rejoice, O thou that art sinful, thy Saviour offereth thee pardon! Rejoice also, O thou Gentile, God calleth thee to life! For the Son of God, when the fulness of the time was come, which had been fixed by the unsearchable counsel of God, took upon Him the nature of man, that He might reconcile that nature to Him Who made it, and so the devil, the inventor of death, is met and beaten in that very flesh which hath been the field of his victory.
When our Lord entered the field of battle against the devil, He did so with a great and wonderful fairness. Being Himself the Almighty, He laid aside His uncreated Majesty to fight with our cruel enemy in our weak flesh. He brought against him the very shape, the very nature of our mortality, yet without sin. Heb. iv. 15. His birth however was not a birth like other births for no other is born pure, nay, not the little child whose life endureth but a day on the earth. To His birth alone the throes of human passion had not contributed, in His alone no consequence of sin had had -part. For His Mother was chosen a Virgin of the kingly lineage of David, and when she was to grow heavy with the sacred Child, her soul had already conceived Him before her body. She knew the counsel of God announced to her by the Angel, lest the unwonted events should alarm her. The future Mother of God knew what was to be wrought in her by the Holy Ghost, and that her modesty was absolutely safe.
Therefore, dearly beloved brethren, let us give thanks to God the Father, through His Son, in the Holy Ghost: Who, for His great love wherewith He loved us, hath had mercy on us and, even when we were dead in sins, hath quickened us together with Christ, Eph. ii. 4, 5, that in Him we might be a new creature, and a new workmanship. Let us then put off the old man with his deeds (Col. iii. 9); and, having obtained a share in the Sonship of Christ, let us renounce the deeds of the flesh. Learn, O Christian, how great thou art, who hast been made partaker of the Divine nature, 2 Pet. i. 4, and fall not again by corrupt conversation into the beggarly elements above which thou art lifted. Remember Whose Body it is Whereof thou art made a member, and Who is its Head, (1 Cor. vi. 15.) Remember that it is He That hath delivered thee from the power of darkness and hath translated thee into God's light, and God's kingdom.
Christ is born! Glorify Him!