Our Savior, dearly-beloved, was born today: let us be glad. ... Let the saint
exult in that he draws near to victory. Let the sinner be glad in that he is
invited to pardon. Let the Gentile take courage in that he is called to life.
For the Son of God in the fullness of time ... has taken on him the nature of
[humanity], thereby to reconcile it to its Author: in order that the inventor
of death, the devil, might be conquered through that which he had conquered.
And in this conflict undertaken for us, the fight was fought on great and
wondrous principles of fairness; for the Almighty Lord enters the lists with
his savage foe not in his own majesty but in our humility, opposing him with
the same form and the same nature, which shares indeed our mortality, though it
is free from all sin.
Leo the Great, from Sermon XXI, Feast of the Nativity