ISB ~ Matthew 3 v15

it is fitting:Jesus is sinless and has no need for John’s baptism (Heb 4:15; 1 Pet 2:22). He nevertheless submits to the rite to identify with sinners and align himself with God’s plan. Jesus performs Old Covenant regulations to fulfill and perfect them in the New (5:17; cf. Lk 2:21-28; CCC 536).

• Mystically (St. Thomas Aquinas, ST 3, 39, 8), Jesus’ baptism prefigures the Christian sacrament. The water, Spirit, and divine voice signify the effects of Baptism whereby the soul is cleansed (Acts 22:16), the grace of the Holy Spirit is imparted (3:11; 1 Cor 12:13), and the recipient is adopted as a beloved child of God (3:17; Gal 3:26-27; CCC 537).