Question: Why do Catholics confess their sins to a priest?
Answer: Most Christians recognize that we must "fess up to" or acknowledge our sins in order for God to extend His forgiveness. The Bible is rich with verses that support this, such as in 1 John 1:9, "If we confess our sins, he is faithful and just, and will forgive our sins and cleanse us from all unrighteousness" and Psalm 32:5, "I acknowledged my sin to thee, and I did not hide my iniquity; I said, "I will confess my transgressions to the LORD"; then thou didst forgive the guilt of my sin."
Catholics believe this as well, but we believe God has set up a particular way in which He wishes for this to happen.
After Jesus' resurrection from the dead he appeared to the twelve apostles while they were gathered together in a room. After showing them the nail marks in his hands and the place where the spear pierced his side, he said the following:
"Jesus said to them again, "Peace be with you. As the Father has sent me, even so I send you." And when he had said this, he breathed on them, and said to them, "Receive the Holy Spirit. If you forgive the sins of any, they are forgiven; if you retain the sins of any, they are retained." John 20:21-23Jesus actions here remind us of the promise he made previously to Peter:
"I will give you the keys of the kingdom of heaven, and whatever you bind on earth shall be bound in heaven, and whatever you loose on earth shall be loosed in heaven." Matthew 16:19The Catholic Church believes only God can forgive sin. As we see from the Bible, one of Jesus' primary -- and most controversial! -- activities during his earthly ministry was forgiving people's sins. However, since Jesus would not always be with his Church visibly, we believe the Scriptures indicate that by the power of the Holy Spirit he gave this special authority to the apostles so that the Church could continue to act tangibly in his place.
As the Church grew due to the apostles' evangelizing far away nations, they appointed certain people -- bishops -- to lead individual churches in their absence (and eventual death). By the laying on of hands the bishops of the churches were given the same spiritual authority that was given the apostles so that they could forgive in a visible way the sins of those Christians in their flock.
By the 2nd century Christianity was expanding to the point where bishops could no longer minister personally to all the Christian churches in their region. This required that they appoint pastors to shepherd each church under their authority, carrying out the basic duties of saying Mass, forgiving sin, ministering to the sick, visiting the imprisoned, etc. They called these pastors "presbyters," or priests.
The early Church practiced confession to the bishop or priest just as Catholics do today. The early Christian writings indicate, however, that certain publicly known sins (such as deserting the Church to join a heretic group) were confessed in front of the entire congregation! Eventually the act of confessing one's sins, regardless of what they may be, became a totally private matter between the person and the priest. This remains true today. In fact, a priest may never reveal what is said to him as a confession. Outside the confessional a priest may not even speak to a person about confessed sins (unless the person brings them up on his own) or even indicate that he has heard a particular person's confession. If a priest ever deliberately violates this rule, he is excommunicated from the Church and therefore cannot continue in his duties. Even secular law now recognizes the "privilege of the confessional" in many places (although in the past some priests were imprisoned and even killed for not revealing someone's sins at the demand of the government).
It is important to stress that Catholics do not and have never believed it is the priest or bishop himself that forgives someone's sins, but it is Jesus acting through them, by the authority of the Holy Spirit which he himself has chosen to give them. The bishops and priests are simply performing a service for Christ -- a service which Christ himself indicated he wished to have fulfilled. While it is the priest's voice a Catholic hears when the priest says, "I absolve you from your sins, in the name of the Father, and of the Son, and of the Holy Spirit..." it is Jesus Christ himself that is saying those words.
Another thing to note is that this means of obtaining God's forgiveness is only "necessary" for very serious sins. The Catholic Church calls these "mortal sins," or sins which lead to spiritual death. The Bible speaks of this distinction here:
"If any one sees his brother committing what is not a mortal sin, he will ask, and God will give him life for those whose sin is not mortal. There is sin which is mortal; I do not say that one is to pray for that. All wrongdoing is sin, but there is sin which is not mortal." 1 John 15:16-17
When we commit mortal sins, we are spiritually cut off from God and the Church. The Catholic Church describes it as "destroying charity in the heart of man by a grave violation of God's law." These grave sins include things like abortion, murder, adultery, homosexual acts, witchcraft, apostasy, and so on. To obtain forgiveness and come back into communion with the Church, we must confess our sins by the normal means which Christ has instituted: that is, confession to a priest.
Lesser sins, which of course are still serious and injure us spiritually but do not totally cut us off from God, are called "venial sins." Saying a harsh word when we're tired or sick, failing to pray as much as we should because we're feeling lazy or depressed, being impatient, etc. The Catholic Church does not require that these sins be confessed to a priest, but teaches that we can simply acknowledge them in our daily prayers and be forgiven. This said, the Church still encourages us to mention these venial sins during formal confession with a priest -- especially if we find they are habitual. The reason for this is because confession is not only the means by which God extends His grace and forgiveness to a person, but it also helps form one's conscience. When we hear our sins out loud, and especially when someone else hears them as well, we become more aware of where our imperfections lie -- and thus are more compelled to correct them. The other advantage a Catholic has by confessing his venial sins in the presence of a priest is that the priest can offer him direction and advice on how to refrain from those sins in the future.
Many of the most holy and devout Catholics in ages past made it a practice to confess their venial sins on a regular and frequent basis, and they all testified to the great graces it brought to their spiritual lives. So while it is not absolutely required for a Catholic to go to a priest with their more minor transgressions, it is still considered a good and helpful practice that can help us refrain from those sins more than if we did not confess them at all.
I hope that helps explain why Catholics confess their sins to a priest! If anyone is interested, here are some links that provide additional and more in-depth information on the subject:
What Did The Early Christians Say About Confession? (quotes)
Protestant Objections to Confession (And Catholic Response)
More Protestant Objections to Confession (And More Catholic Answers)
What The Official Catholic Catechism Says About Confession
A great basic book about Confession is: Lord Have Mercy: The Healing Power of Confession by Scott Hahn
The information provided here is, to the best of my ability and knowledge, in accordance with official Catholic teaching. I submit all things said here to the authority of the Catholic Church.