Miller himself is a churchgoing Roman Catholic, a professor of religion at Juniata College in Huntingdon, Pa., and author of "Born Divine: The Birth of Jesus and Other Sons of God." He argues that "all Biblical scholars who practice the historical critical study of the Bible understand the Nativity story is a combination of legend and early Christian storytelling."

Mary's virginity at the time of conception was not a universal belief during the early Christian era, Miller says. Biblical scholars who question the literalness of the Nativity story point to the gospels of Mark and John, which do not mention Jesus' paternity. If an angel had actually visited Mary to proclaim that "the Holy Ghost shall come upon thee" to conceive a son (Luke 1:35), they argue, wouldn't all the Gospels have written about it? (These same critics also point to other details that may be embellishments: There is no historical evidence, for example, that the Roman Emperor Caesar Augustus conducted a census that would have propelled Mary and Joseph to travel to Bethlehem.)

First century Christians, Miller says, were comfortable with Roman and Greek biographies of what he calls "the wonder boys," including Alexander the Great, Caesar Augustus and Plato. The only way the ancients could explain the accomplishments of men like this, Miller says, was to create a mythology that made them part human, part divine. All were said to have human mothers but to have been sired by gods such as Apollo.

To convince their First century pagan and Jewish audiences to become Christians, did Matthew and Luke spin the story of Jesus' birth to mirror the Greek and Roman birth stories? Did they add the detail of Mary's virginity (one that sets it apart from other stories of divine fatherhood) as another layer of proof that Jesus was the son of God? Or maybe as a way to underscore Mary's purity?

In pagan birth stories, Miller says, "the gods assumed human form and slept with the women and impregnated them. Luke was trying to move away from that kind of crass connotation."

Mary's virginity also echoed the prophecy of Isaiah — "Behold, a virgin shall conceive, and bear a son, and shall call his name Emmanuel" — which underscored the Christian belief that Jesus was the predicted Messiah. Some Biblical scholars argue that the original Hebrew should have been more properly translated as "young girl" rather than virgin. McClenahan of the Salt Lake Theological Seminary counters that the word (almah) is best translated as "young woman who is unattached," and therefore, in the context of the times, "it would be assumed, if she were to be pregnant, it would either be immoral or a miracle." The meaning of the passage, therefore, requires it to mean "virgin," he says.