The doctrine of the virgin birth is one of the foundations and central doctrines of true Christianity. Without it, Christ would be just a "mere man" born of Mary and not the begotten Son of God. If he were anything short of literally God in the flesh, his death would not be able to atone for the sins of all humans who either did or would exist.
If Christ were brought into this world as the result of Joseph having sex with Mary, the entire New Testament would be a lie. This is why belief in a virgin birth is so critical. It is a unique Biblical miracle that testifies of his divine origin.
Most Christians, up until the nineteenth century, accepted the Virgin Birth without hesitation until liberal theology began to challenge the Bible's miracles and the divinity of Christ. Liberal theologians sought to reinterpret Jesus as nothing more than a wise man and good teacher who died in the first century A.D. They sought to cast doubt on the veracity of the Bible and its miracles like Jesus' entry into the world through a virgin.
Isaiah 7:14 is the only verse in the Old Testament where the miraculous conception and birth of man's Savior is predicted. It states, "Behold, the virgin shall conceive and bear a Son, and shall call His name Immanuel (Isaiah 7:14, NKJV).
The focus of the attack against the supernatural conception of Christ through Mary centers on the translation of Isaiah 7:14. The Hebrew word translated "virgin" in the verse is almah (Strong's #H5959). Some have suggested that almah does not mean a young chaste woman. They state that Isaiah would have used the Hebrew word bethulah (Strong's #H1330) if he desired to unambiguously write about a young woman who never had sex.
The truth is that almah is the BEST word the prophet Isaiah could have chosen to convey the concept of virginity. There exists, in the Old Testament, no place where almah requires clarification before it can be translated as referencing a young female of marriageable age. This is not the case with the word bethulah, which requires clarification (see Genesis 24:16).
The Hebrew word almah is used in only six other places in the Old Testament other than Isaiah 7:14. In all cases, it references a virgin or 'maid' who is at an age where she can be married and give birth to children. This is not the case with bethulah, which can refer to a married female (Joel 1:8).
Mary, while she was a virgin betrothed to Joseph, became pregnant with Jesus through the power of the Holy Spirit (Matthew 1:20, Luke 1:31, 34 - 35). An angel was sent to Joseph to tell him that Mary's condition did not come through sex but that she would give birth to Christ through a miracle from God (Matthew 1:20 - 21).
Matthew then quotes Isaiah 7:14, stating "Behold, the virgin (Greek parthenos, Strong's #G3933) shall be with child and shall give birth to a son, and they shall call His name Emmanuel" (Matthew 1:22 - 23, HBFV).
The Greek word parthenos is used in eleven other places in the New Testament besides Matthew 1:23 and Luke 1:27. It is consistently translated as "virgin" or its plural and always refers to someone who is chaste physically or in two cases considered so spiritually (see 2Corinthians 11:2, Revelation 14:4).
The Incarnation, or birth of Christ through Mary, was one of the most anticipated events in human history. Those living in Jerusalem knew the time of the promised Christ was near (Luke 2:38). Parthian Magi diligently watched for the manifestation of the King of Kings and rejoiced when they were miraculously led to Him who deserves worship (Matthew 2:1 - 2).
The religious leaders at the temple also awaited the Messiah and knew he would be born in Bethlehem (Matthew 2:4 - 6). The virgin birth through Mary of the One named Immanuel (God WITH us) began the Eternal's glorious plan to redeem us from sin and offer us a way to live in love forever.