I. The Incarnation an unceasing source of joy
Though all the divine utterances exhort us, dearly beloved, to “rejoice in the Lord always,” yet today we are no doubt incited to a full spiritual joy, when the mystery of the Lord's nativity is shining brightly upon us, so that we may have recourse to that unutterable condescension of the Divine Mercy, whereby the Creator of men deigned to become man, and be found ourselves in His nature whom we worship in ours. For God the Son of God, the only-begotten of the eternal and not-begotten Father, remaining eternal “in the form of God,” and unchangeably and without time possessing the property of being no way different to the Father He received “the form of a slave” without loss of His own majesty, that He might advance us to His state and not lower Himself to ours. Hence both natures abiding in possession of their own properties such unity is the result of the union that whatever of Godhead is there is inseparable from the manhood: and whatever of manhood, is indivisible from the Godhead.
Source: Sermons (New Advent)