22 But to prayer we must also join penance, the spirit of penance, and the practice of Christian penance. Thus Our divine Master teaches us, whose first preaching was precisely penance: "Jesus began to preach and to say, Do penance" ( Matth . iv. 17). The same is the teaching of all Christian tradition, of the whole history of the Church. In the great calamities, in the great tribulations of Christianity, when the need of God's help was most pressing, the faithful either spontaneously, or more often following the lead and exhortations of their holy Pastors, have always taken in hand the two most mighty weapons of spiritual life: prayer and penance. By that sacred instinct, by which unconsciously as it were the Christian people is guided when not led astray by the sowers of tares, and which is none other than that "mind of Christ" (I Cor . ii. 16) of which the Apostle speaks, the faithful have always felt immediately in such cases the need of purifying their souls from sin with contrition of heart, with the sacrament of reconciliation, and of appeasing divine Justice with external works of penance as well.
Source: Caritate Christi Compulsi (Vatican.va)