Our Saviour, dearly-beloved, was born today (Christmas day): let us be glad. For there is no proper place for sadness, when we keep the birthday of the Life, which destroys the fear of mortality and brings to us the joy of promised eternity. No one is kept from sharing in this happiness. There is for all one common measure of joy, because as our Lord, the destroyer of sin and death, finds none free from the charge, so He comes to free us all.
How do we miss this obvious fact in all the standard Protestant hymns we hear this time of year? Christ is born, God becomes man, so He can destroy the fear of death. The joy we should feel at this time is the joy that we are now promised eternal life with God in His kingdom. Jesus comes not as a baby, but as the destroyer of sin and death.
Saint Leo continues,
Let the saint exult in that he draws near to victory. Let the sinner be glad in that he is invited to being pardon. Let the gentile take courage in that he is called to life. For the Son of God in the fullness of time which the inscrutable depth of the Divine counsel has determined, has taken on him the nature of man, thereby to reconcile it to its Author: in order that the inventor of death, the devil, might be conquered. And in this conflict undertaken for us, the fight was fought on great and wondrous principles of fairness.
Why are we so joyful at this time of year? God came to defeat Satan and liberate us from the chains of death. What does this have to do with gift giving, with the parties, with the "Christmas Spirit." A warrior is born who is a peacemaker, bringing us all peace in our hearts from the gift he bestows on us. Without the fear of death we can now willingly sacrifice our own self-will out of selfless love to do God's will and show our love for others as well as God. We gain the ability to live His commandments through becoming a member of His body as we are joined with Him in Baptism and are nurtured regularly as we take His Body and Blood into our body through Holy Communion. It is the Incarnation that makes all this possible. Let's properly glorify Him in this great Feast of the Church.
Sain Leo says,
Christian, acknowledge your dignity, and becoming a partner in the Divine nature, refuse to return to the old baseness by degenerate conduct. Remember the Head and the Body of which you are a member. Remember that you were rescued from the power of darkness and brought out into God's light and kingdom. By the mystery of Baptism you were made the temple of the Holy Ghost: and do not subject yourself once more to the devil; because your purchase money is the blood of Christ, because He shall judge you in truth Who ransomed you in mercy, who with the Father and the Holy Spirit reigns forever and ever.
Reference: Sermons on the Feast of the Nativity of Christ by St Leo the Great, Sermon 21