The doctrine of the virgin birth, or perhaps more accurately the virgin conception, is important for many reasons. On it hang the doctrines of original sin, the inspiration of Scripture, who Jesus was, and what Jesus did through salvation. Deny the virgin birth and you deny these other doctrines as well.
Scripture describes the event for us. In response to Mary’s question, “But how can this happen? I am a virgin” (Luke 1:34), Gabriel says “The Holy Spirit will come upon you, and the power of the Most High will overshadow you...” (Luke 1:35). Matthew 1:20 says "…For the child within her was conceived by the Holy Spirit.”
From these passages, it’s clear Jesus’ birth was the result of the Holy Spirit working within Mary’s body. The immaterial (the Holy Spirit) and the material (Mary’s womb) were both involved. Mary of course could not impregnate herself, and was simply a vessel. Only God could perform the miracle of the Incarnation. We should also note that the virgin birth displays the tri-unity of God. The Spirit gives conception to Mary, carrying out the Father’s plan of the virgin birth of the Son.
However, denying a physical connection between Mary and Jesus would imply Jesus was not truly human. Scripture teaches that Jesus was fully human, with a physical body. This He received from Mary. At the same time Jesus was fully God with an eternal, sinless nature (John 1:14; 1 Timothy 3:16; Hebrews 2:14-17).
Romans 5 indicates the sin nature is passed down from generation to generation through the father. But Jesus was not born in sin, He had no sin nature (Hebrews 7:26). He was the Holy One of God since He was conceived by the Holy Spirit. Because He was the Son of God, He partook of the divine nature.
The virgin birth circumvented the transmission of the sin nature and allowed the eternal God to become a perfect man. He never sinned, which qualified Him to be a righteous substitutionary sacrifice for sinners. Christ was fully man—therefore able to die—and fully God, so His death is of infinite worth and applicable to any and all who believe.
The virgin birth is also important because it affirms the inspiration and authority of Scripture. If we deny the teaching and say it’s a myth, then we undermine the rest of the Bible. Isaiah 7:14 says, “All right then, the Lord himself will give you the sign. Look! The virgin will conceive a child! She will give birth to a son and will call him Immanuel (which means ‘God is with us’).” The word "virgin" is the Hebrew word almah, which is translated as virgin or young woman.
Those who deny the inspiration of Scripture say this word should be translated as "young woman," and the disciple Matthew was reading into the word to suit his agenda (Matthew 1:23).
Centuries before Christ, when the Hebrew Old Testament was translated into Greek by Jewish rabbis (70 of them according to tradition), the Greek word they chose to render almah was parthenos, which means a literal virgin. Centuries before there was any Christian agenda around to influence the story, the expectation among the Greek-speaking Jews (at a minimum) was that a virgin would conceive and bear a son. This is how Matthew understood Isaiah’s words.
Finally, the virgin birth is important because it shows us that salvation must be from God. It shows we have no part in undoing the sin curse. Only He can save us. Humanity couldn’t produce its own redeemer—salvation came from outside us. Salvation is supernatural and from the Lord.