It is through the sacrament of Penance that the baptized can be reconciled with God and with the Church:
Penance has rightly been called by the holy Fathers "a laborious kind of baptism." This sacrament of Penance is necessary for salvation for those who have fallen after Baptism, just as Baptism is necessary for salvation for those who have not yet been reborn.
Source: Catechism of the Catholic Church (Vatican.va)