After all the people had been baptized and Jesus also had been baptized and was praying, heaven was opened and the Holy Spirit descended upon him in bodily form like a dove. And a voice came from heaven, “You are my beloved Son; with you I am well pleased.” (Lk 3:15-16, 21-22)
https://bible.usccb.org/bible/readings/010922.cfm
In the Gospel of Luke, the revelation of Jesus as Son of God occurs at the Baptism, which of course begins his public ministry. The Antiphon for today is Hodie Christus Natus Est (Today Christ is Born): Today Christ is born; today the Savior has appeared; today the Angels sing, the Archangels rejoice; today the righteous rejoice, saying: Glory to God in the highest. Alleluia!