The Apostle's Creed associates faith in the forgiveness of sins not only with faith in the Holy Spirit, but also with faith in the Church and in the communion of saints. It was when he gave the Holy Spirit to his apostles that the risen Christ conferred on them his own divine power to forgive sins: "Receive the Holy Spirit. If you forgive the sins of any, they are forgiven; if you retain the sins of any, they are retained."
(Part Two of the catechism will deal explicitly with the forgiveness of sins through Baptism, the sacrament of Penance, and the other sacraments, especially the Eucharist. Here it will suffice to suggest some basic facts briefly.)
Source: Catechism of the Catholic Church (Vatican.va)