In the second reading, St. Peter instructs us that:
In truth, I see that God shows no partiality. Rather, in every nation whoever fears him and acts uprightly is acceptable to him.-- Acts 10:34-35
That Jesus should be baptized by St. John the Baptist is odd: Why should Our Lord need baptism? He doesn't. But He descends into the waters with us, to fulfill all righteousness, in radical solidarity with us sinners whom He came, Himself without sin, to save.
Many ancient tales speak of a katabasis, of the hero's descent to the underworld. Baptism, too, is a kind of death and rebirth:
You were buried with him in baptism, in which you were also raised with him through faith in the power of God, who raised him from the dead.-- Colossians, 2:12
In Christ's descent into death in baptism in solidarity with us sinners, perhaps we see a glimpse of St. Peter's insight into His saving grace for "every nation," i.e., a sign of the Harrowing of Hell.