Why do Catholics confess their sins to a priest?

Because on Easter evening Jesus breathed the Holy Spirit on the apostles and told them that whose sins they forgave were forgiven and whose sins they retained were retained, and a man cannot forgive or retain what he has not been told. The text is John 20:21-23. Jesus appears to the apostles, says "as the Father has sent me, I also send you," breathes on them, and says "receive the Holy Spirit. Whose sins you shall forgive, they are forgiven them, and whose sins you shall retain, they are retained." Look at what is happening in the Greek. The breathing is an aorist, a decisive and obvious act, not a symbolic gesture. "Receive" is an aorist imperative, a permanent bestowal. And the forgiving clause is a conditional clause of expectancy, which means Jesus is anticipating that people will confess their sins to the apostles and the apostles will in turn forgive them as if Jesus himself had done it. The sacrament is in that verse, and the Church has read it that way for two thousand years. Now, why must the sins be spoken aloud to a man? Because of the second half of the verse. Jesus gives the apostles the power to retain sins as well as forgive them. Retention requires judgment, and judgment requires knowledge. If the priest has no idea what you did, he cannot possibly decide whether to absolve or withhold. The power Jesus handed over is useless unless the penitent opens his mouth. This is the same logic as Matthew 16:19, where Jesus gives Peter the keys and says whatever you bind on earth shall be bound in heaven and whatever you loose on earth shall be loosed in heaven. John 20:23 is the more specific fulfillment of that grant. The Catholic Church is the only institution on the face of the earth that can take a man in mortal sin and send him out cleansed in his soul, and she can do it because Christ said so, not because she invented it. Someone always raises James 5:16, "confess your sins to one another," as though it refuted us. It does not. James is dealing with sins we commit against each other, and the point there is that you go to the person you wronged so that he can forgive you and you can be healed of the resentment between you. That is Matthew 18:15-17 territory, brotherly correction, and it is good and necessary. But notice what James says two verses earlier in James 5:14-15: if anyone is sick, let him bring in the presbyters of the church, let them pray over him and anoint him with oil, and if he be in sins, they shall be forgiven him. James himself puts the forgiving of sins in the hands of the ordained, not the congregation. The two verses sit side by side and do two different jobs. Acts 8 is worth looking at too. Simon the magician sins grievously after his baptism. Peter tells him to repent and pray, and Simon answers, "pray for me, that none of these things which you have spoken come upon me." That is very nearly a confessional. The penitent names his sin, and then seeks the priest's mediation with God so the sin can be absolved. In Acts 5, Peter gives Ananias and Sapphira the chance to confess their sin to him, they refuse, and by Peter's own word their lives are taken. That is retention. Paul gives us the theology of the office in 2 Corinthians 5:18-20. God has reconciled us to himself through Christ and has given to us, Paul says, the ministry of reconciliation and the word of reconciliation, and therefore we are ambassadors for Christ. Take the word "ambassador" seriously. An ambassador does not manufacture the terms of peace, he carries and applies the terms his sovereign set. And do notice what Paul says next, because it bears on how the sacrament works: the atonement of Christ did not automatically take away men's sins. Christ opened the pathway back to God. Men still have to walk it, prompted by grace, by repenting. Confession is the appointed place where that repentance gets applied to an actual sinner with an actual name. Practically, here is how I explain it. Confession was instituted for mortal sin. Adultery, murder, theft, the serious things. You confess everything you know of, and if you genuinely forget a sin, God is not standing there with a hammer, you can tell him later. Venial sins you may bring to the priest, and it is a good practice, but you are not bound to. Mortal sin is different. Except in a genuine emergency, where there is no priest and no realistic prospect of one, you do not get to hand your mortal sin to God directly and assume the transaction cleared. And when you do go, expect the priest to talk straight with you. That is also his job. Sin is serious business, and the last thing he should want is for you to walk back out still in it. Never take God's forgiveness for granted. It is always an act of grace that he forgives you at all.