The doctrine of the Virgin Birth of Jesus holds that Jesus' birth was the result of a miraculous conception whereby the Virgin Mary conceived a baby in her womb by the power of the Holy Spirit, without a human father. Christ's miraculous birth tells us much about his nature. That He was born of woman demonstrates that He was indeed human and became one of us. Christ's humanity, however, was not precisely the same as our own. We are born with original sin, Christ was not.
The Virgin Birth also relates to the deity of Christ. While it is certainly possible for Deity to enter the world in a manner other than a virgin birth, the miracle of his birth points to Christ's divinity. The announcement of the angel Gabriel to Mary underscores this point. When he told Mary she would have a son, Mary was perplexed: "How can this be, since I do not know a man?" (Luke 1:34).
Gabriel's answer to Mary is of decisive significance for our understanding of the Virgin Birth: "The Holy Spirit will come upon you, and the power of the Highest will overshadow you; therefore, also, that Holy One who is to be born will be called the Son of God" (Luke 1:35). Moments later the angel added, "For with God nothing will be impossible" (Luke 1:37).
Aside from artificial insemination, which is a modern, non-miraculous variation on conception, nothing is more regular or commonplace in nature than the normal causal relationship for the conception of a baby. For a woman to become pregnant who has not had sexual intercourse with a man is not only biologically extraordinary, it is clearly against the laws of nature.
But Mary's child was not generated by Mary, herself. The "father" of the baby is the Holy Spirit. The language of the Spirit's coming upon Mary and "overshadowing" her echoes the descriptive account of the Holy Spirit's work in the original creation of the world. It reveals that this baby will be a special creation with His father being God Himself.
Those who do not believe in the Virgin Birth usually do not believe that Jesus is the true Son of God. Thus, the Virgin Birth is a watershed doctrine, separating orthodox Christians from those who do not believe in the Resurrection and Atonement.