When I was an Evangelical Protestant thinking about returning to the Catholic Church of my baptism, among Catholic practices I found difficult to accept was the sacrament of penance.
I thought that the sacrament took away from Christ’s sufficiency to forgive all our sins, past, present, and future. For I believed that it diminished the scope of Christ’s atonement if I had to do something in order to acquire forgiveness, such as confess to a priest and/or do penance, i.e., say some prayers, do a good deed, or engage in a spiritual discipline, after receiving absolution. But good reasons eventually led me to change my mind.
First, even Evangelical Protestants have a means for dealing with post-baptismal sin: the rededication. The backslider, depending on the severity of his iniquity, rededicates his life to Christ by walking the aisle once again, as he did when he first converted. In fact, given the Protestant understanding of justification and sanctification – that good works and good living follow from being truly saved – the backslider may wonder if his first confession was a sham. So, he confesses again. Thus, it became clear to me that Christianity requires some way to deal with post-baptismal sin
Second, the New Testament speaks often of post-baptismal sin. In the Lord’s Prayer, Jesus mentions the need for ongoing repentance in the believer’s life (Mt. 6:12). St. John writes to baptized Christians: “If we say that we have no sin, we deceive ourselves, and the truth is not in us. If we confess our sins, he who is faithful and just will forgive us our sins and cleanse us from all unrighteousness.” (I Jn. 1:8-9). Moreover, St. Paul (I Cor. 6:9-10; Gal. 5:19-21; Eph. 5:3-15; Col. 3:5-10), St. John (1 Jn. 5:16-17), and the author of Hebrews (Heb. 13:4-5) warn believers of sins that are mortal, i.e., if the Christian commits them he risks losing what Catholics call “sanctifying grace.” Thus, Christianity by its very nature requires a ministry of reconciliation. (2 Cor. 5:14-20).
Third, I read the Catechism , and it quickly disabused me of my sacramental ignorance. “Doing penance,” I learned, is not a work the Church requires in order to guarantee forgiveness. For Christ’s death is “the ransom that would free men from the slavery of sin.”  And when one confesses and receives absolution in the sacrament, one’s sins are forgiven by God, for “only God forgives sins .” “But,” and this is key, “it does not remedy all the disorders sin has caused .”
Penance helps configure us to Christ, who alone expiates our sins
So, for example, if I confess to a priest that I have stolen $500 from my neighbor, I will receive absolution. Although I am forgiven for my offense, for penance, I must at least return my neighbor’s property. But all sins (including theft) diminish, and sometimes rob us of, our spiritual health. For this reason, acts of penance “help configure us to Christ, who alone expiated our sins once and for all. They allow us to become co-heirs with the risen Christ, `provided we suffer with him.’ [Rom. 8:17; Rom. 3:25; I Jn. 2:1-2] ” And given the Church’s understanding of operating and cooperating grace , even the merit we acquire from our penance is a work of God’s grace.
Fourth, the New Testament speaks of fasting, praying, and spiritual discipline as means by which one may become better equipped and more disposed to holiness in one’s Christian journey (Mt. 6:1-8, 16-18; Acts 13:2-3; Acts 14:23; 1 Cor. 9:25-27; 2 Cor. 1: 4-6;Col 4:2; Eph. 6:18; 1 Thess. 4:4-8; 1 Tim. 4:7-8; 2 Tim. 1:17; Jas. 4:8-10; I Pet. 4:7). In fact, the author of Hebrews writes of the reality of God’s fatherly discipline of us, his sons, in the formation of our souls (Heb. 12:5-13).
Fifth, the Scripture teaches that the Church is integral to the ministry of reconciliation. Christ came to Earth to forgive sins (Mt. 9:6), and after his resurrection he imparted to his followers that same power (Jn. 20:21-23). In Mt. 18, in the context of the administration of Church discipline, Jesus gives his disciples the power of binding and loosing. (Mt. 18:18). James encourages the sick to seek out the Church’s presbyters so that they may “pray over him, anointing him with oil in the name of the Lord; and the prayer of faith will save the sick man, and the Lord will raise him up; and if he has committed sins, he will be forgiven. Therefore confess your sins to one another, and pray for one another, so that you may be healed. The prayer of the righteous is powerful and effective.” (5:14-15—emphasis added)
Given its biblical foundation, it’s not surprising that the sacrament begins to develop early in Christian history. According to Protestant historian J. N. D. Kelly , by the beginning of the third century, the ecclesial and theological elements on which today’s private confession is based – that the penitent confesses to the Church that not only has the power to absolve him of his sins but also the power to impose penance on the penitent – are already in place. And there is no shortage of passages from the Church Fathers  that clearly show that penance was an integral and uncontroversial part of the sacramental infrastructure of the Christian life.
So the development of the sacrament is not surprising at all. And the fact that Western and Eastern Rite Catholics as well as the Eastern Orthodox practiced it without controversy until the time of the Reformation made it impossible for me to think that penance was not a legitimate Christian practice consistent with biblical theology.
Now that I’ve returned to the Church, I wish more of my fellow Catholics understood the great gift we have in Confession.