Institutes of the Christian Religion (1559) - John Calvin
Book 3 - How We Receive the Grace of Christ
Chapter 4 - Impurity of Sophists' doctrine of repentance;
discussion of confession and satisfaction
Section 28 - Venial and mortal sins
Rome says some sins are venial, not deserving death, requiring lesser penance.
Rom 6:23; Ezek 18:20.
All sins of believers are venial, as the punishment of death is removed.
Rom 8:1; Ps 32:1-2.
While you are making satisfaction for some venial sin, you will commit numerous others. "For not a day passes when the most righteous of men does not fall time and again." Prov 24:16.
Section 29 - Forgiveness of sins involves remission of penalty
Rome says guilt is gone after forgiveness, but a penalty (of God's justice) remains to be paid. "Though I believe I have already more than fully confirmed this [is false], I shall add certain other testimonies by which these wriggling snakes may be so held fast that after this they will be unable to coil up even the tip of their tail."
Jer 31:31, 34; Ezek 18:21-22, 24 - to not remember sin means to not demand penalty.
Isa 38:17; 44:22; Micah 7:19; Ps 32:1-2; Isa 1:18; 50:20.