But we have to be careful that we understand what that means, or we could easily throw out a great deal of Christian teaching and literature that emphasizes the mortification of sin or mourning over sin, thinking that stuff is all the same as penance. What I have described as 'penance' is not how everyone would use the term, but it is the most common perception today. In contrast, the mortification of sin or the pursuit of holiness is not self-punishment, but a quest to purge the heart of sinful desires. Quite a few Christians have written powerfully over the years of the need to prick our own consciences and rend our hearts over the wrong desires within us and the sins we are prone to commit. Richard Sibbes, in his immensely comforting work The Bruised Reed, nonetheless says:
There is a dangerous slighting of the work of humiliation, some alleging this for a pretence for their casual dealing with their own hearts, that Christ will not break the bruised reed; but such must know that every sudden terror and short grief is not that which makes us bruised reeds… but a working our hearts to such a grief as will make sin more odious unto us than punishment, until we offer a ‘holy violence’ against it. (Sibbes, Chapter 2)
By ‘humiliation’ Sibbes does not mean ‘embarrassment’ or putting ourselves to shame. He means the work of humbling ourselves; the act of putting off all of our pride and our desire to look good, and instead being very vulnerable and honest before God about our faults. (This is, in fact, how 'penance' has been understood by some saints in Christian tradition, but it is so often interpreted as I have described it so far that I think it is clearer to draw the distinction between penance as self-punishment and mortification of sin as purification.) One of the things that transformed Martin Luther's understanding of Christianity from a religion demanding we work off our sins under the fear of punishment into embracing the Gospel of justification by faith alone was learning from Erasmus "that the Greek word metanoeite meant ‘to repent’ not ‘to do penance’” – for example, in Matthew 4:17. "[T]his insight was reflected at the first of his 95 Theses." (See Timothy George, Erasmus Before the Storm.)
"Offer to God a sacrifice of thanksgiving, and perform your vows to the Most High, and call upon me in the day of trouble; I will deliver you, and you shall glorify me.” (Psalm 50:14-15).
Third, penance implies that if you make yourself feel bad enough, or punish yourself enough, God will forgive you. This is a gross perversion of the Gospel. God does not forgive you because you have beat yourself up enough to convince Him you’ve paid for your mistake. God forgives you because Christ already took the punishment for your mistake. There is nothing more you can add to that. Your efforts to “work off” the guilt of your sin are pointless, and worse, they are prideful or distrustful. Either you doubt that Christ’s death really paid for all your sins, and therefore think you have to do more to pay them off (as if the cross wasn’t enough), or you have a prideful desire to be independent, which makes you resist the idea that you can’t add anything to Christ’s sacrifice. You want to pay some of the debt yourself so you feel like you earned your way and did your share.