The Bible says in Isaiah that God would give us a sign: "The virgin shall conceive and bear a Son" (Isaiah 7:14). The word in Hebrew that we translate as "virgin" is almah, and it can mean "virgin or young woman." The word has been translated "virgin," however, because there is nothing unusual about a young woman giving birth, so that would be no sign at all.
But in the New Testament, the word that is used for the Greek translation of almah is parthenos, and that clearly means virgin. It has no other meaning except virgin. The Bible tells us that there was a young virgin named Mary. The angel of the Lord came to Mary and said, "The Holy Spirit will come upon you, and ... Overshadow you" (Luke 1:35). He went on to tell her that a child would be formed in her, and she would give birth to the Messiah.
This is the way God entered into humanity to bring a second Adam. God the Son enfleshed Himself—the Holy Spirit bringing about conception, parthenogenesis, the virgin beginning—without the intervention of the normal reproductive cycle of man. That is why Jesus is called the Son of God. He was not the son of Joseph, and He was not the son of a Roman soldier. He was not the son of any human father. He was conceived by the Holy Spirit.
God brought about virtually a second creation, a second man, without the original sin of the male line from Adam. Linking Jesus into the family tree of Mary, however, made him a descendant of David and Abraham, which fulfilled the various promises that God had made to them. Further, it was from His mother, Mary, that the Lord received His human nature. So Jesus Christ was unique, conceived by the Holy Spirit, but of "the house and lineage of David" (Luke 2:4).