The mystery kept secret for long ages, promised through His prophets in the holy Scriptures, is today revealed (see Romans 16:25-26).
This is the "Gospel of God" that Paul celebrates in today's Epistle—the good news that "God is with us" in Jesus Christ. The sign promised to the House of David in today's First Reading is given in today's Gospel. In the virgin found with child, God himself has brought to Israel a savior from David's royal line (see Acts 13:22-23).
Son of David according to the flesh, Jesus is the Son of God, born of the Spirit. He will be anointed with the Spirit (see Acts 10:38), and by the power of Spirit will be raised from the dead and established at God's right hand in the heavens (see Acts 2:33-34; Ephesians 1:20-21).
He is the "King of Glory" we sing of in today's Psalm. The earth in its fullness has been given to Him. And as God swore long ago to David, His Kingdom will have no end (see Psalm 89:4-5).
In Jesus Christ we have a new creation. Like the creation of the world, it is a work of the Spirit, a blessing from the Lord (see Genesis 1:2). In Him, we are saved from our sins, are called now "the beloved of God."
All nations now are called to belong to Jesus Christ, to enter into the House of David and Kingdom of God, the Church. Together, through the obedience of faith, we have been made a new race—a royal people that seeks for the face of the God of Jacob.
He has made our hearts clean, made us worthy to enter His holy place, to stand in His presence and serve Him.
In the Eucharist, the everlasting covenant is renewed, the Advent promise of virgin with child—God with us—continues until the end of the age (see Matthew 28:20; Ezekiel 37:24-28).