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Creed › … › Paragraph 2. "Conceived by the Power of the Holy Spirit and Born of the Virgin Mary"
The Annunciation to Mary inaugurates "the fullness of time", The time of the fulfilment of God's promises and preparations. Mary was invited to conceive him in whom the "whole fullness of deity" would dwell "bodily". The divine response to her question, "How can this be, since I know not man?", was given by the power of the Spirit: "The Holy Spirit will come upon you."
Source: Catechism of the Catholic Church (Vatican.va)