Proverbs 16:6 reads: "By loyalty and faithfulness iniquity is atoned for." This atonement has nothing to do with the eternal punishment due to sins (hell), but rather temporal punishment, which remains after sins are forgiven. Making temporal atonement for sins is called penance. Penances can be done by patiently enduring the sufferings that come one's way, or they can be self-inflicted. Catholics do not do penance to make up for their eternal debt, for this is impossible. They do it to satisfy for the temporal punishment due for sins that have already been forgiven.
Protestants argue that if Christ's sufferings were sufficient than it is useless to do penance. The fact is, God does not wipe out all our eternal and temporal punishments in one fell swoop through the sinner's prayer, as many Protestants hold. God leaves us with a debt of temporal punishment (2 Sam. 12:13-14; Num. 20:12), which is atoned for by the merits of Christ when we do penance. So Christ's sufferings are sufficient, but they are applied to us over time as we atone for our sins.
Why does God require temporal punishment? God requires temporal punishment partly as a punishment, and to correct our ways. So Hebrews says: "‘My son, do not regard lightly the discipline of the Lord, nor lose courage when you are punished by him. For the Lord disciplines him whom he loves' . . . God is treating you as sons; for what son is there whom his father does not discipline? If you are left without discipline . . . then you are illegitimate children and not sons. Besides this, we have had earthly fathers to discipline us and we respected . . . He disciplines us for our good . . . all discipline seems painful rather than pleasant; later it yields the peaceful fruit of righteousness to those who have been trained by it" (Heb. 12:5-13).
The Biblical practice of atoning for temporal punishment is found in numerous places in the Old Testament: "Then the Israelites, all the people, went up to Bethel, and there they sat weeping before Yahweh. They fasted that day until evening" (Judg. 20:26). "When Ahab heard these words, he . . . put on sackcloth and fasted . . . ‘Because he has humbled himself, I will not bring this disaster in his day'" (1 Kgs. 21:27-29). "So we fasted and petitioned our God about this, and he answered our prayer" (Ezra 8:21-23). "And all the people . . . humbled their souls in fastings and prayers . . . And the priests put on haircloths, and they caused the little children to lie prostrate before the temple of the Lord . . . that their children might not be made a prey, and their wives carried off, and their cities destroyed, and their holy things profaned" (Jud. 4:7-9). "So I turned to the Lord God and pleaded with him in prayer and petition, in fasting, and in sackcloth and ashes" (Dan. 9:3). Through penance, a believer humbles himself in order to appease God's wrath.
Various forms of penance, especially fasting, is clearly endorsed by the Scriptures: "When you fast, put oil on your head and wash your face, so that it will not be obvious to men that you are fasting, but only to your Father, who is unseen; and your Father, who sees what is done in secret, will reward you" (Matt. 6:16-18). "Paul and Barnabas appointed elders for them in each church and, with prayer and fasting, committed them to the Lord, in whom they had put their trust" (Acts 14:23). (See also Acts 13:2; Jas. 4:8-10). "And I will give power to my two witnesses, and they will prophesy for 1,260 days, clothed in sackcloth" (Rev. 11:3).
The Didache (Didache 7:1, 8:1 [A.D. 80]) "Before the baptism, let the one baptizing and the one to be baptized fast, as also any others who are able. Command the one who is to be baptized to fast beforehand for one or two days. . . . you shall fast on Wednesday and Friday".
Clement (Letter to the Corinthians 57 [A.D. 80]) "You, therefore, who laid the foundation of the rebellion, submit to the presbyters and be chastened to repentance, bending your knees in a spirit of humility".
Ignatius of Antioch (Letter to the Philadelphians 3 [A.D. 110]) "For as many as are of God and of Jesus Christ are also with the bishop. And as many as shall, in the exercise of penance, return into the unity of the Church, these, too, shall belong to God, that they may live according to Jesus Christ".
Cyprian of Carthage (Letters 9:2 [A.D. 253]) "Sinners may do penance for a set time, and according to the rules of discipline come to public confession, and by imposition of the hand of the bishop and clergy receive the right of communion".
Jerome (Commentary on Ecclesiastes 10:11 [A.D. 388 ]) "If the serpent, the devil, bites someone secretly, he infects that person with the venom of sin. And if the one who has been bitten keeps silence and does not do penance, and does not want to confess his wound . . . then his brother and his master, who have the word [of absolution] that will cure him, cannot very well assist him".
Augustine (Sermon to Catechumens on the Creed 7:15, 8:16 [A.D. 395]) "For those whom you see doing penance have committed crimes, either adultery or some other enormities. That is why they are doing penance. If their sins were light, daily prayer would suffice to blot them out. . . . In the Church, therefore, there are three ways in which sins are forgiven: in baptisms, in prayer, and in the greater humility of penance".