And so our Lord Jesus Christ, being at birth true man though He never ceased to be true God, made in Himself the beginning of a new creation, and in the form of His birth started the spiritual life of mankind afresh ....

What mind can grasp this mystery, what tongue can express this gracious act?  Sinfulness returns to guiltlessness and the old nature becomes new; strangers receive adoption and outsiders enter upon an inheritance.  The ungodly begin to be righteous, the miserly benevolent, the incontinent chaste, the earthly heavenly.  And whence comes this change, save by the right hand of the Most High?  For the Son of God came to destroy the works of the devil, and has so united Himself with us and us with Him that the descent of God to man's estate became the exaltation of man to God's.

-- St. Leo the Great, Nativity Sermon VII