Our Lord tied the forgiveness of sins to faith and Baptism: "Go into all the world and preach the gospel to the whole creation. He who believes and is baptized will be saved." Baptism is the first and chief sacrament of forgiveness of sins because it unites us with Christ, who died for our sins and rose for our justification, so that "we too might walk in newness of life."
Source: Catechism of the Catholic Church (Vatican.va)