In adoring the birth of our Saviour, we find we are celebrating the commencement of our own life. For the birth of Christ is the source of life for Christian folk, and the birthday of the Head is the birthday of the body.
Although every individual that is called has his own order, and all the sons of the Church are separated from one another by intervals of time, yet -- as the entire body of the faithful being born in the font of baptism is crucified with Christ in His passion, raised again in His resurrection, and placed at the Father's right hand in His ascension -- so with Him are they born in this nativity. ...
For unless He came down to us in this humiliation, no one would reach His presence by any merits of his own.
-- St. Leo the Great, Nativity Sermon 6.