Paenitentiam Agere §1

¹ Doing penance for one's sins is a first step towards obtaining forgiveness and winning eternal salvation. That is the clear and explicit teaching of Christ, and no one can fail to see how justified and how right the Catholic Church has always been in constantly insisting on this. She is the spokesman for her divine Redeemer. No individual Christian can grow in perfection, nor can Christianity gain in vigor, except it be on the basis of penance.