The Forgiveness of Sins

“BY these seven Sacraments we receive the remission of sins,(1) and so in the Creed there follows immediately “the remission of sins.” The power was given to the Apostles to forgive sins. We must believe that the ministers of the Church receive this power from the apostles; and the apostles received it from Christ; and thus the priests have the power of binding and loosing. Moreover, we believe that there is the full power of forgiving sins in the Church although it operates from the highest to the lowest, i.e., from the Pope down through the prelates.” (2)

~St. Thomas Aquinas: Exposition of the Apostles’ Creed. 
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Notes: 

(1) Baptism and Penance are called Sacraments of the dead, because they take away sin and give the first grace of justification. The other five Sacraments are called Sacraments of the living, because one who receives them worthily is already living the life of grace. But the Sacraments of the living produce the first grace when the subject, guilty of a grievous fault, approaches the Sacraments in good faith, that is to say, with invincible ignorance of his fault, and with attrition. (Pourrat, Theology of the Sacraments; 1914 ed.)

(2) “For Our Lord did not give the power of so sacred a ministry to all, but to the bishops and priests only. The same must be said regarding the manner in which the power is to be exercised; for sin can be forgiven only through the Sacraments, when duly administered. The Church has received no power otherwise to remit sins. Hence it follows that in the forgiveness of sins both priests and Sacraments are, as it were, the instruments which Christ, Our Lord, the Author and giver of salvation, makes use of to accomplish in us pardon of sin and the grace of justification.” (Roman Catechism)

Seven Sacraments Altarpiece (left panel detail), by Rogier van der Weyden. Oil on oak panel, 1445-50;Koninklijk Museum voor Schone Kunsten, Antwerp.