St Thomas is thinking of the case of an adult coming to baptism without any intention of leading a good life. The conditions of the baptism of John (Luke iii, 10-14) hold likewise of Christ's baptism: so St Peter, metanoêsate kai baptisthêtô hekastos (Acts ii, 38). But these are preliminaries, not parts of baptism; whereas the contrition and confession of the penitent are as true sacramental parts of the Sacrament of Penance as the absolution of the priest.

Of God and His Creatures: 4.72