Fr. Bob Stull explains the baptism of Jesus was not just something to model for Christians (there weren't any at the time!), but demonstrated a distinct change of the priestly order from the High Priests of the line of Aaron to the line of Melchizedek in the tribe of Judah (kingdom monarchs). (cf. Psalm 11, Hebrews 3:13-17, Matthew 3, Luke 3). Jesus is fully authorized to both forgive sins of the people, and rule as their King.