After tracing Jesus' ancestry back in history to the oaths sworn by God to come and bring forgiveness of sins, the miracle of the virgin birth is yet another sign that God had indeed come down not to punish His people for their sins, but had joined His people to save them from their sins.
It is His people that He came to bring salvation from sin. Who then are His people who are saved from their sins?
Christ was eternally the Son of God and His entrance into the family of humankind was by the miraculous work of the Holy Spirit.
In much the same way, entrance for us into the family of God is by the miraculous work of the Holy Spirit, given to all who trust in Jesus to forgive and free them from their sin.