Section — Wouters, Manual of Moral Theology
Section I. On the Existence of the Sacrament of Penance
I. Penance is a true sacrament of the New Law; this proposition is of faith from the Council of Trent: "If anyone says that in the Catholic Church Penance is not truly and properly a sacrament... instituted by Christ our Lord, let him be anathema."1
Note. As regards the time of institution of this sacrament, the Council of Trent teaches:2 "But the Lord instituted the sacrament of Penance especially3 then, when, raised from the dead, he breathed upon his disciples, saying: 'Receive the Holy Spirit; whose sins you shall forgive...' By so signal a deed and by words so clear, the consensus of all the Fathers has always understood that the power of remitting and retaining sins, for reconciling the faithful fallen after Baptism, was communicated to the Apostles and to their legitimate successors."
II. Penance is a sacrament really distinct from the sacrament of Baptism; this proposition likewise is of faith from the Council of Trent: "If anyone, confounding the sacraments, says that Baptism itself is the sacrament of Penance... let him be anathema."4
3 It is said "especially" because Christ had already promised the sacrament earlier (Matthew 16:19 and 18:18).