Blessed Virgin Mary
One of the hardest things for the nonbeliever to accept about Christmas is the virgin birth, that Mary was a virgin when she gave birth to the Christ Child. To be fair, many liberal theologians abandoned that belief long ago, too, to the point it became a dividing line in the faith. The Fundamentalist movement of the early twentieth-century listed the virgin birth as one of the "fundamentals" of the faith; if you didn't believe it, you couldn't properly be considered a Christian. Is it really as important as all that?
Short answer: yes.
Long answer: From Old Testament prophecies to the New Testament gospels, the virgin birth is a crucial piece of doctrine. If Mary were not a virgin, Joseph would have been right to stone her for adultery. The gospel accounts, both in Matthew 1 and Luke 1-2, would be simply wrong. Jesus would lose all claim to being the only-begotten Son of the Living God, having as he would two perfectly human parents. Only God can forgive sins; a human death on the cross would have been meaningless in the story of salvation. Jesus must be the Son of God, must have divine parentage, and that requires conception by the Holy Spirit and the virgin birth -- precisely as the Bible states.
There are a few hills to die on when it comes to the facts about our faith, and this is one of them. If we abandon the virginity of Mary at the Nativity, we abandon our hope of redemption; it truly is that important, that fundamental to the Christian religion. Defend this truth, even in our modern age, but defend it with love. That is what the babe in the manger would want us to do.