Virgin birth of Jesus
The virgin birth of Jesus is a the Christian belief that Jesus was born, not through normal sexual intercourse between a man and a woman, but through a miracle where the Holy Ghost impregnated Mary's uterus directly, and that she gave birth to Jesus before ever having sex. This belief comes from stories found in the Gospel of Matthew and another found in the Gospel of Luke, which Christians believe fulfills the prophecy described in the Book of Isiah. The belief that Jesus was born of a virgin was adopted by most Muslims as a virgin birth is found in the Quran, though they don't view Jesus as a part of their god. The virgin birth is extremely important to Christian theology, but most scholars now admit that the event has a weak historical foundation.
Book of Isaiah (800-580 BCE)
This is the prophecy Christians believe was fulfilled with the virgin birth.
Isaiah 7:10-16, NIV.
Early Epistles (50-66 CE)
The earliest writings about the birth of Jesus are from the epistles attributed to Paul which describe nothing but a natural human birth. Apologists attempt to inject a virgin birth into these passages, but their attempts are without evidence.
Galatians 4:4-5, NIV (circa 50 CE).
Romans 1:1-4, NIV (circa 56 CE).
Philippians 2:5-8, NIV (circa 62 CE).
Gospel of Mark (66-70 CE)
When Jesus is first mentioned in Mark he is already an adult; there is nothing mentioned of his birth.
Gospel of Matthew (80-90 CE)
Matthew 1:18-25, NIV.
Gospel of Luke (80-100 CE)
Luke 1:26-38, NIV.
Gospel of John (90–110 CE)
Like Mark, the Gospel of John begins with Jesus already an adult and doesn't mention his birth.
Later Epistles (80+ CE)
None of the epistles written, either at the time of Matthew or Luke or after, even suggest a virgin birth.
Christians interpret the virgin birth story as a fulfillment of the prophecy in Isaiah. While some of the more liberal denominations suggest the virgin birth could be a metaphor, most Christians take the passages literally, and, since humans aren't normally born from virgins, view it as a miracle.
The only historical evidence for the virgin birth of Jesus comes from the gospels of Matthew and Luke. Each author is anonymous and neither claims or even implies they are an eye-witness to any part of Jesus' birth and neither describes the source of their account or how they received it. Matthew's author describes knowledge of conception being presented to Joseph in a dream while Luke's author describes it being presented to Mary; neither claims or implies other witnesses. Both sources describe Jesus being born in Bethlehem to Mary and Joseph without claiming or implying other witnesses. Both sources were written around 80-90 years after the event supposedly took place, meaning anyone old enough to be a reliable witness would most likely be dead, and anyone capable of writing the gospels would, at best, have been a young child when the event took place.
However, the fact that there are two at least partially-independent sources cannot be ignored. Most historians agree that Matthew and Luke shared common sources (as suggested by the various hypothetical solutions to the Synoptic Problem), though none of the popular hypotheses place the virgin birth neatly in any one shared source. My current hypothesis is that an anonymous person who was familiar with the Septuagint translation of Isaiah, in an effort to disavow Docetism, appropriated the prophecy and concocted the virgin birth, and this idea reached both the authors of Matthew and Luke who expanded on it for their respective gospels. I hold this hypothesis only because it fits the existing evidence, and freely admit it has no basis in evidence itself.
While I'm more inclined to believe historical events when there are multiple independent sources, I find severe faults with the authors of Matthew and Luke and don't feel that they have given extraordinary evidence for their extraordinary claims. My reasons are as follows:
The Hebrew word for "virgin" is bĕthuwlah, but the author of Isaiah uses the word `almah, which Hebrew scholars agree translates simply to "young woman" and has nothing to do with virginity. For example, Proverbs 30:18-20 describes an adulterous woman, who can't possibly be a virgin, as an `almah. This means that Isaiah's prophecy is properly translated to the mundane, "the young woman shall conceive, and bear a son," which is precisely how the Jewish Publication Society translates it. However, the authors of Matthew and Luke might have used the Greek Septuagint as their source, and in the Septuagint, the Hebrew word `almah is incorrectly translated to the Greek parthenos, which means "virgin." Being born of a virgin is certainly interesting, and may be why both authors fixated on it, but their fixation appears to be the result of a mistranslation.
Furthermore, while Christians assume that Isaiah's prophecy is symbolic and applies to an event that will take place centuries after the prediction, Jewish scholars of Isaiah agree the passage was meant to be taken literally and refers to a child that was born shortly after the prediction just as the kingdoms fell. This means that neither Matthew nor Luke have historical foundation for a virgin birth (see "The Gospel of Matthew," by R.T. France, p.56-57).
Being born of a virgin is indeed interesting, yet the earliest Christian authors either make no mention of Jesus' birth at all or state that it was entirely ordinary. Apologists suggest Paul implicitly wrote about a virgin birth, but if you read the passages in the Early Epistles above, you'll see there is nothing there.
The virgin birth stories found in Matthew and Luke are markedly different and the differences are only compounded through the Nativity of Jesus. Every New Testament author writing after Matthew and Luke fail to acknowledge the virgin birth story and even the authors of Mark and John, two sources which focused specifically of the events of Jesus' life, didn't bother to mention it. A common apologist response is that the various authors who don't mention the virgin birth either purposely left it out because they didn't feel it was important or didn't know about it. I don't find either excuse believable. As authors of books purporting to have an intimate knowledge of Jesus, how could they possibly not know about his miraculous birth, and, assuming they knew, why would they possibly leave out what early Christians would think of as such an obvious fulfillment of prophecy?