28 Our first need is for internal repentance; the detestation, that is, of sin, and the determination to make amends for it. This is the repentance shown by those who make a good Confession, take part in the Eucharistic Sacrifice and receive Holy Communion. The faithful should be specially encouraged to do this during the novena to the Holy Spirit, for external acts of penance are quite obviously useless unless accompanied by a clear conscience and the detestation of sin. Hence Christ's severe warning: "Unless you repent you will all perish in the same manner."(26) God forbid that any of Our sons and daughters succumb to this danger.
Source: Paenitentiam Agere (Vatican.va)