The Virgin Birth
Jesus was born of a Virgin. This is consistent with the Christian Doctrine of Salvation and accepted as fact. Jesus could have come in other ways, but this is how He came.
What happened way back there in history before the start of the first century AD? A virgin, Mary, was engaged to a man named Joseph. Unmarried women were referred to as "virgins" because it was assumed that they were. If you weren't a virgin at marriage, then you were a social outcast and your husband could divorce you.
The doctrine of the virgin birth of Jesus is fundamental to Christian Theology. I say "doctrine" because most Christians accept it as non-negotiable fact today. Doctrine because, in Christian belief, it is an essential part of God's formula for dealing with sin in humanity.
The virgin birth was foretold 700 years before Christ's birth by the prophet Isaiah. "Therefore the Lord Himself will give you a sign: Behold, the virgin shall conceive and bear a son, and shall call his name Immanuel" (Isaiah 7:14) NKJV. In the New Testament Gospels of Matthew and Luke, this prophecy is fulfilled. The virgin birth is written of in the Bible as fact.
"Now in the sixth month the angel Gabriel was sent by God to a city of Galilee named Nazareth to a virgin betrothed to a man whose name was Joseph, of the house of David. The virgin's name was Mary…Then the angel said to her, 'Do not be afraid, Mary, for you have found favor with God. And behold, you will conceive in your womb and bring forth a Son, and shall call His name JESUS…Then Mary said to the angel, 'How can this be, since I do not know a man?' And the angel answered and said to her, '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 1:26,27,30,31,34,35) NKJV
These Gospel authors were contemporaries of Jesus and Mary. Matthew was one of the twelve apostles and an eyewitness to much of what Jesus said and did. Luke was a doctor and a Greek who wrote from the viewpoint of Mary. Both writers give careful genealogies, one from Joseph and the other from Mary. Nevertheless, the virgin birth is understood despite the careful inclusion of the family tree in the Gospel. The genealogy proves the fulfillment of Scripture regarding Jesus ancestry, but doesn't preclude the miracle of a virgin birth.
The doctrine of the virgin birth has been with the Church since it's infancy. Ignatius wrote early in the second century: "For our God, Jesus the Christ, was conceived in the womb by Mary, according to a dispensation, of the seed of David but also of The Holy Ghost".
Why was a virgin birth necessary? Jesus was not a newly conceived person such as the rest of us are at conception. Jesus was God becoming a human being, an Incarnation. In order for God to take on human form required divine intervention. God is the Father of Jesus. This is a miracle, which can't be explained.
Jesus had to be virgin born because of his sinless nature. Christian teaching says that Jesus lived a sinless life. If He were the son of human beings he would have inherited the sin nature. If He had sinned or had the sin nature He could not have been the perfect sacrifice, the Lamb of God.
Many people have a problem with the virgin birth. The greatest problem is probably that it is, indeed, a miracle. If you believe in miracles, then the Incarnation is one. If we can believe that Jesus fed 5,000 with a few loaves and fish, or walked on water, or healed many sick then His virgin birth is only another miracle. God becoming man is a miracle in itself!
Some say that modern science knows this is impossible. The facts of life have not changed in 2,000 years. Joseph wanted to divorce Mary when he found out she was pregnant. He knew that there is only one way to have a baby. It was a miracle, a miracle of God.
Basic understanding of Christian Theology renders belief in the virgin birth perfectly consistent with the whole Bible story of sin and redemption. In the Old Testament, the Israelites sacrificed perfect lambs (lambs without blemish) to God as atonement for sin. In the New Testament "The Lamb of God" is Jesus, the perfect sin sacrifice offered once for all. Part of why he fulfills the requirement is that he was sinless, or perfect. God could have come in other ways but this is the way that He chose to come.