The grace of Christ alone provides true satisfaction for sin and peace to the conscience.

Christ has provided full satisfaction. But such is their perversity, they say that both forgiveness of sins and reconciliation take place once for all when in Baptism we are received through Christ into the grace of God; that after Baptism we must rise up again through satisfactions; that the blood of Christ is of no avail, except in so far as it is dispensed through the keys of the church. And I am not speaking of a doubtful matter, since not one or another, but all the Schoolmen, have, in very clear writings, betrayed their own taint. For their master, after he confessed that Christ on the tree paid the penalty of our sins, according to Peter’s teaching [1 Peter 2:24], corrected that statement by adding the exception that in Baptism all temporal penalties of sins are relaxed, but after Baptism they are lessened by the help of penance, so that the cross of Christ and our penance may work together. But John speaks far differently: “If anyone has sinned, we have an advocate with the Father, Jesus Christ …; and he is the propitiation for our sins” [1 John 2:1–2]. “I am writing to you, little children, because your sins are forgiven in his name.” [1 John 2:12] Surely he is addressing believers, to whom, while he sets forth Christ as the propitiation of sins, he shows that there is no other satisfaction whereby offended God can be propitiated or appeased. He does not say: “God was once for all reconciled to you through Christ; now seek for yourselves another means.” But he makes him a perpetual advocate in order that by his intercession he may always restore us to the Father’s favor; an everlasting propitiation by which sins may be expiated. For what the other John said is ever true: “Behold the Lamb of God, who takes away the sins of the world!” [John 1:29; cf. John 1:36]. He, I say, not another, takes them away; that is, since he alone is the Lamb of God, he also is the sole offering for sins, the sole expiation, the sole satisfaction. For while the right and power of forgiving sins properly belong to the Father, in which respect he is distinguished from the Son, as we have already seen, Christ is here placed on another level because, taking upon himself the penalty that we owe, he has wiped out our guilt before God’s judgment. From this it follows that we shall share in the expiation made by Christ only if that honor rest with him which those who try to appease God by their own recompense seize for themselves.

—John Calvin
Institutes, 3.4.26.