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Creed › … › Paragraph 2. "Conceived by the Power of the Holy Spirit and Born of the Virgin Mary"
Mary's virginity
The Gospel accounts understand the virginal conception of Jesus as a divine work that surpasses all human understanding and possibility: "That which is conceived in her is of the Holy Spirit", said the angel to Joseph about Mary his fiancee. The Church sees here the fulfilment of the divine promise given through the prophet Isaiah: "Behold, a virgin shall conceive and bear a son."
Source: Catechism of the Catholic Church (Vatican.va)