The Incarnatus within the Nicene Creed has been translated “by the power of the Holy Spirit He became incarnate from the Virgin Mary, and was made man.” The Incarnation is the belief that the divine nature of the second person of the Trinity is united (“And the Word became flesh, and dwelt among us”) with a human nature in one being, Jesus Christ, who was both “truly God and truly man”. The Incarnation is celebrated each year at Christmas, but it is also celebrated at the Feast of the Annunciation (March 25), nine gestational months before Jesus’ ceremonial birthday. At the Annunciation, the archangel Gabriel announced to Mary that she would conceive and become the mother of Jesus, the Son of God, marking his Incarnation. For many centuries until the reforms of Vatican II, a genuflection was made at the Incarnatus during the Creed of every Catholic mass to reverence the gift of the Incarnation to humankind, and this reverence continues to be practiced in Anglo-Catholic parishes. Because of its implications, it is not surprising that genuflections at the Incarnatus were commonly practiced by Christians at Christmas and also on the Feast of the Annunciation.
~Dr. Gil Haas, St. Augustine of Canterbury Episcopal Church