The Old Testament book of Isaiah, written approximately 600 years before Jesus’ birth, contains this prophecy:
“Therefore the Lord himself will give you a sign. Behold, the virgin shall conceive and bear a son, and shall call his name Immanuel (God with us).”
When taken in context of the passage, this appears to be referring to a more immediate sign given to the king at the time of the prophecy. Many will argue that the Hebrew word for “virgin” in this passage could just as easily be translated “young woman.” Be that as it may, the book of Matthew, written by a Hebrew, seems to take this prophecy as referring to Christ when he says, “All this took place to fulfill what the Lord had spoken by the prophet: “Behold, the virgin shall conceive and bear a son, and they shall call his name Immanuel”(which means, God with us).”
Assuming all this to be true, what was the purpose of the virgin birth? The most obvious answer is that, if Christian doctrine is correct, and Jesus is both God and Man, then Mary contributed the human part while God contributed the divine part. However, there is an argument that could be made that Jesus could only be pure and sinless by virtue of the virgin birth.
When God is giving the curses in the garden of Eden, he tells the serpent that he will "put enmity between you and the woman, between your seed and her seed." This is the one and only reference in the Bible to the "seed" of a woman. In every other instance, "seed" in the sense of offspring, is always credited to the man. Catholics have a doctrine which states that Mary was sinless. While Protestant doctrine does not grant Mary any such quality, the reason Catholics believe this is interesting, mainly that they do not believe that God could mingle with that which is in any way corrupt.
While it could be argued that Jesus was sinless because he was God, he was also human. Mary was not just an incubator for a foreign biological entity, Jesus shared a genetic component with Mary. This is seen throughout the Old Testament, where Abraham, then Isaac, then Jacob, then Judah, and finally David were promised that the Messiah would come from their line of descent.
Now it's entirely possible that God could have purified the genetic material that Mary contributed to the birth, but this is assuming that sin is a genetic quality and not a quality of imputed guilt. Paul makes the statement that through one man, Adam, sin came into the world. This is interesting considering that Eve was the first to sin, but Adam is credited with passing this guilt on to the human race.
Finally, the fact that Jesus was born of a virgin woman seems significant. God didn't simply purify Joseph's genes and allow the two to conceive. Rather, he left the man out of the process entirely and caused a virginal woman to conceive. Theologians make a distinction in Christ's sinlessness. They say that he is "not able to sin." This refers to his divine nature, that no sin is present in God. But they also say that he is "able not to sin." This refers to his human nature. According to the Bible, humans have no capacity to avoid sinning. It is part of their nature. But Jesus' nature included the capacity to resist sin.
And this may well be the reason for the virgin birth. These facts appear to argue that sin nature is imputed through the father, not the mother. Because Jesus’ father was divine and sinless, he could be born of a human mother and still remain sinless.