from Pope St. Leo the Great:
Dearly beloved brethren:
Unto us is born this day a Savior. Let us rejoice. It would be unlawful to be sad today, for today is Life's birthday; the birthday of that Life which, for us mortal creatures, takes away the sting of death and brings the bright promise of an eternal hereafter. It would be unlawful for any man to refuse sharing in our rejoicing. All men have an equal part in the great reason why we are joyful, for our Lord, who is the destroyer of sin and death, finding that all are bound under condemnation, is come to make all free.
Rejoice if you are a saint, for you are drawing nearer your crown! Rejoice if you are a sinner, for your Savior offers you pardon! And if you are a pagan, rejoice, for God calls you to life!
For when the fullness of time was come the Son of God took upon Himself the nature of man so that He might reconcile that nature to Him who made it; hence the devil, the inventor of death, is met and conquered in that very flesh which had been the field of his victory.
Let us give thanks to God the Father through His Son in the Holy Spirit, who for His great love wherewith He loved us has had mercy on us and has quickened us together with Christ even when we were dead in sins, that in Him we might be a new creature and a new handiwork.
Let us then put off the old man with his deeds, and having obtained a share in the sonship of Christ, let us renounce the deeds of the flesh. Be conscious, O Christian, of your dignity! You have been made partaker of the divine nature; do not fall again by a corrupt manner of life into the beggarly elements above which you are lifted. Remember whose body it is of which you are a member, and who is the Head. Remember that it is He who has delivered you from the power of darkness, and transferred you into God's light and God's kingdom.
[usually read at Matins upon Christmas Day]