29 But the faithful must also be encouraged to do outward acts of penance, both to keep their bodies under the strict control of reason and faith, and to make amends for their own and other people's sins. St. Paul was caught up to the third heaven—he reached the summit of holiness—and yet he had no hesitation in saying of himself "I chastise my body and bring it into subjection."(27) On another occasion he said: "They who belong to Christ have crucified their flesh with its passions and desires."(28) St. Augustine issued the same insistent warning: "It is not enough for a man to change his ways for the better and to give up the practice of evil, unless by painful penance, sorrowing humility, the sacrifice of a contrite heart and the giving of alms he makes amends to God for all that he has done wrong."(29)
Source: Paenitentiam Agere (Vatican.va)