Confession of Sin to a Priest
The practice of Confession arises from the example and command of Jesus, who showed that human nature could be used by God as an instrument of grace and forgiveness. He said "That you may know that the Son of Man has the power to forgive sin..." (Mt. 9:6; Mk 2:7-10; Lk 5:21-24). The Hebrew title He used was "ben Adam" meaning "Son of Adam." This was the Hebrew way of saying "a human being." Jesus always gloried in His Humanity, since through It He redeemed us. He communicated this authority to His Apostles on Easter night, "Whose sins you shall forgive they are forgiven, whose sins you shall retain they are retained" (Jn 20:19-23). In this way He gave the Apostles the power to give "Peace" (v.21), which is nothing less than the reconciliation of man with God.
The text even makes clear how Confession is to be conducted. Christ's representative, the priest, must decide whether to forgive or retain. Therefore, the penitent must confess each and every serious sin, that is anything which separates him from Christ. If the priest judges he is truly sorry, He must absolve since Christ's Passion merited forgiveness for every repentant sinner. Only if the person shows no willingness to give up sin does the priest retain, that is withhold absolution, as we "do not give what is holy to dogs" (Mt 7:6).
In one form or another the Sacrament of Penance has been in continuous practice in the Church. Its existence in all the Churches of the First Millenium, even those separated from Rome, shows its apostolicity. The present Catholic discipline of secret confession dates to the early middle ages, though there are suggestions of an even earlier practice. Prior to that, confession of sins involved lengthy public penance for great sins such as adultery, murder and apostasy from the faith. Thankfully, it is much easier today. The point was, however, that serious sin is a horrendous offense against God that ought to be rare among the baptized but frequently is not. In the second and third century theological battles were fought over whether Penance could be received more than once after Baptism. The rigorists, like Tertullian, left the Church and their movements passed into history. Even the practice of the sacrament today is no encouragement to sin, as they thought. On the contrary it requires humility to confess your sins. It also gives great peace to hear the priest say in Jesus' name "I absolve you from your sins in the name of the Father and of the Son and of the Holy Spirit." It is Christ's will that we hear those words.
Although God CAN forgive sin directly it requires a perfect motive: love of Him and sorrow over having offended such a good Lord. An imperfect motive would mean we have not fully turned from our sin back to God. Not every one can rise to the occasion, so rather than excluding the marginal person struggling with sins, perhaps even over a lifetime, Christ has given us the Sacrament in which He raises us up, even when our sorrow is weak and imperfect. This shows the dependence of the sacrament on grace and mercy. As Jesus himself said, He came not to save the self-righteous but the sinner.
