Isaiah 7:14, Therefore, the Lord Himself will give you a sign. Behold, the virgin will conceive and bear a son, and shall call His name Immanuel.
The birth of Jesus was a sign. We have lost the understanding of what a sign means. It is not a symbol, as evangelicals would have you believe. Biblically, a sign from God is the assurance that God has done or is doing something. The rainbow was a sign to Noah that God shall surely not wipe out all creation via a flood again; this covenant was already done.
Thus, what does the sign of Jesus’ birth—Jesus’ advent—mean? He was the sign that God actually walked among us (Immanuel = “God with us”). He was the sign that God is now bringing His people back to Him as well as every nation (cf. Isaiah 9:2-3; 40:1-5; 42:1-7; 56:6-8). In Christ, God was and is doing these things.
As the sign of God’s promise, Jesus Christ delivers that promise to us by grace through faith (Ephesians 2:8-9). As a sign, Jesus actually does something, namely, He dies for you, forgives your sins, and rose from the dead for you. He mediates between you and God the Father (1 Timothy 2:5)—He Himself brings your prayers before God the Father, as does the Holy Spirit (Romans 8:27). Jesus is God’s promise for you.