Now very many persons see the base tricks, deceits, thefts, and greediness with which the indulgence traffickers have heretofore mocked and beguiled us, and yet they do not see the very fountain of the impiety itself. As a consequence, it behooves us to indicate not only the nature of indulgences but also what in general they would be, wiped clean of all spots. The merits of Christ and the holy apostles and martyrs our opponents call the “treasury of the church.” They pretend that the prime custody of this storehouse, as I have already hinted, has been entrusted to the Bishop of Rome, who controls the dispensing of these very great benefits, so that he can both distribute them by himself and delegate to others the management of their distribution. Consequently, plenary indulgences, as well as indulgences for certain years, stem from the pope; indulgences for a hundred days, from the cardinals; and of forty days, from the bishops!
Now these, to describe them rightly, are a profanation of the blood of Christ, a Satanic mockery, to lead the Christian people away from God’s grace, away from the life that is in Christ, and turn them aside from the true way of salvation. For how could the blood of Christ be more foully profaned than when they deny that it is sufficient for the forgiveness of sins, for reconciliation, for satisfaction—unless the lack of it, as of something dried up and exhausted, be otherwise supplied and filled? “To Christ, the Law and all the Prophets bear witness,” says Peter, that “through him we are to receive forgiveness of sins.” [Acts 10:43 p.] Indulgences bestow forgiveness of sins through Peter, Paul, and the martyrs. “The blood of Christ cleanses us from sin,” says John [1 John 1:7 p.]. Indulgences make the blood of martyrs the cleansing of sins. “Christ,” says Paul, “who knew no sin, was made sin for us” (that is, satisfaction of sin) “so that we might be made the righteousness of God in him” [2 Corinthians 5:21 p., cf. Vg.]. Indulgences lodge satisfaction of sins in the blood of martyrs. Paul proclaimed and testified to the Corinthians that Christ alone was crucified and died for them [cf. 1 Corinthians 1:13]. Indulgences declare: “Paul and others died for us.” Elsewhere Paul says, “Christ acquired the church with his own blood.” [Acts 20:28 p.] Indulgences establish another purchase price in the blood of martyrs. “By a single offering Christ has perfected for all time those who are sanctified.” [Hebrews 10:14.] Indulgences proclaim: Sanctification, otherwise insufficient, is perfected by the martyrs. John says that “all the saints have washed their robes… in the blood of the Lamb.” [Revelation 7:14.] Indulgences teach that they wash their robes in the blood of the saints.
John Calvin, Institutes of the Christian Religion 3.5.2