Among the fundamental beliefs of Christianity is that Jesus was born to Mary while she was a virgin. Matthew 1:20 tells us “that which is conceived in her (Jesus) is of the Holy Ghost.”
Martin Luther says it well:
“It is a great, unspeakable endowment that we have in faith to regard this as God’s consummate wisdom that he, who created heaven and earth, is born of a virgin. Among Jews and Gentiles this has been judged as a particularly foolish proclamation, when first it was preached to the world. It was considered to be absurd, just as today still so very many regard it to be ridiculous that sublime, divine majesty, God himself, should lower himself so deeply, not only to create, nourish, and sustain mankind, but also himself to become a man. To sum up, human reason does not understand it; the devil, the world, and human reason object to it, exclaiming, ‘Nothing more foolish has ever been foisted on people int he world!’ For this reason we must diligently preach and study this article, so that we become well versed in and strengthened by it, in no way entertaining any doubts about it, but becoming ever more sure that God sent his Son into the world, to become man and be born of a woman. For this is solely the gift and wisdom belonging to us who are Christians, that we are able to say that no greater wisdom, no more sublime truth, has appeared in the world than that God, who created heaven and earth, was born of a virgin, that he, therefore, has such members as eyes, ears, hands, and feet, body and soul, just like any other human being.”
Martin Luther, Preached Christmas Eve, 1532: The Complete Sermons of Martin Luther (Grand Rapids: Baker Books), Vol. 7, p 210.