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Objection 1. It would seem that it is not necessary to confess to a priest. For we are not bound to confession, except in virtue of its Divine institution. Now its Divine institution is made known to us (James 5:16): "Confess your sins, one to another," where there is no mention of a priest. Therefore it is not necessary to confess to a priest.
Objection 2. Further, Penance is a necessary sacrament, as is also Baptism. But any man is the minister of Baptism, on account of its necessity. Therefore any man is the minister of Penance. Now confession should be made to the minister of Penance. Therefore it suffices to confess to anyone.
Objection 3. Further, confession is necessary in order that the measure of satisfaction should be imposed on the penitent. Now, sometimes another than a priest might be more discreet than many priests are in imposing the measure of satisfaction on the penitent. Therefore it is not necessary to confess to a priest.
Objection 4. Further, confession was instituted in the Church in order that the rectors might know their sheep by sight. But sometimes a rector or prelate is not a priest. Therefore confession should not always be made to a priest.
On the contrary, The absolution of the penitent, for the sake of which he makes his confession, is imparted by none but priests to whom the keys are intrusted. Therefore confession should be made to a priest.
Further, confession is foreshadowed in the raising of the dead Lazarus to life. Now our Lord commanded none but the disciples to loose Lazarus (John 11:44). Therefore confession should be made to a priest.
I answer that, The grace which is given in the sacraments, descends from the Head to the members. Wherefore he alone who exercises a ministry over Christ's true body is a minister of the sacraments, wherein grace is given; and this belongs to a priest alone, who can consecrate the Eucharist. Therefore, since grace is given in the sacrament of Penance, none but a priest is the minister of the sacrament: and consequently sacramental confession which should be made to a minister of the Church, should be made to none but a priest.
Reply to Objection 1. James speaks on the presupposition of the Divine institutions: and since confession had already been prescribed by God to be made to a priest, in that He empowered them, in the person of the apostles, to forgive sins, as related in John 20:23, we must take the words of James as conveying an admonishment to confess to priests.
Reply to Objection 2. Baptism is a sacrament of greater necessity than Penance, as regards confession and absolution, because sometimes Baptism cannot be omitted without loss of eternal salvation, as in the case of children who have not come to the use of reason: whereas this cannot be said of confession and absolution, which regard none but adults, in whom contrition, together with the purpose of confessing and the desire of absolution, suffices to deliver them from everlasting death. Consequently there is no parity between Baptism and confession.
Reply to Objection 3. In satisfaction we must consider not only the quantity of the punishment but also its power, inasmuch as it is part of a sacrament. In this way it requires a dispenser of the sacraments, though the quantity of the punishment may be fixed by another than a priest.
Reply to Objection 4. It may be necessary for two reasons to know the sheep by sight. First, in order to register them as members of Christ's flock, and to know the sheep by sight thus belongs to the pastoral charge and care, which is sometimes the duty of those who are not priests. Secondly, that they may be provided with suitable remedies for their health; and to know the sheep by sight thus belongs to the man, i.e. the priest, whose business it is to provide remedies conducive to health, such as the sacrament of the Eucharist, and other like things. It is to this knowledge of the sheep that confession is ordained.
Quotes from the early Church (taken from Catholic.com)
"Confess your sins in church, and do not go up to your prayer with an evil conscience. This is the way of life. . . . On the Lord’s Day gather together, break bread, and give thanks, after confessing your transgressions so that your sacrifice may be pure" (Didache 4:14, 14:1 [A.D. 70]).
The Letter of Barnabas
"You shall judge righteously. You shall not make a schism, but you shall pacify those that contend by bringing them together. You shall confess your sins. You shall not go to prayer with an evil conscience. This is the way of light" (Letter of Barnabas 19 [A.D. 74]).
Ignatius of Antioch
"For as many as are of God and of Jesus Christ are also with the bishop. And as many as shall, in the exercise of penance, return into the unity of the Church, these, too, shall belong to God, that they may live according to Jesus Christ" (Letter to the Philadelphians 3 [A.D. 110]).
"For where there is division and wrath, God does not dwell. To all them that repent, the Lord grants forgiveness, if they turn in penitence to the unity of God, and to communion with the bishop" (ibid., 8).
"[The Gnostic disciples of Marcus] have deluded many women. . . . Their consciences have been branded as with a hot iron. Some of these women make a public confession, but others are ashamed to do this, and in silence, as if withdrawing from themselves the hope of the life of God, they either apostatize entirely or hesitate between the two courses" (Against Heresies 1:22 [A.D. 189]).
"[Regarding confession, some] flee from this work as being an exposure of themselves, or they put it off from day to day. I presume they are more mindful of modesty than of salvation, like those who contract a disease in the more shameful parts of the body and shun making themselves known to the physicians; and thus they perish along with their own bashfulness" (Repentance 10:1 [A.D. 203]).
"[The bishop conducting the ordination of the new bishop shall pray:] God and Father of our Lord Jesus Christ. . . . Pour forth now that power which comes from you, from your royal Spirit, which you gave to your beloved Son, Jesus Christ, and which he bestowed upon his holy apostles . . . and grant this your servant, whom you have chosen for the episcopate, [the power] to feed your holy flock and to serve without blame as your high priest, ministering night and day to propitiate unceasingly before your face and to offer to you the gifts of your holy Church, and by the Spirit of the high priesthood to have the authority to forgive sins, in accord with your command" (Apostolic Tradition 3 [A.D. 215]).
"[A final method of forgiveness], albeit hard and laborious [is] the remission of sins through penance, when the sinner . . . does not shrink from declaring his sin to a priest of the Lord and from seeking medicine, after the manner of him who say, ‘I said, "To the Lord I will accuse myself of my iniquity"’" (Homilies on Leviticus 2:4 [A.D. 248]).
Cyprian of Carthage
"The apostle [Paul] likewise bears witness and says: ‘ . . . Whoever eats the bread or drinks the cup of the Lord unworthily will be guilty of the body and blood of the Lord’ [1 Cor. 11:27]. But [the impenitent] spurn and despise all these warnings; before their sins are expiated, before they have made a confession of their crime, before their conscience has been purged in the ceremony and at the hand of the priest . . . they do violence to [the Lord’s] body and blood, and with their hands and mouth they sin against the Lord more than when they denied him" (The Lapsed 15:1–3 (A.D. 251]).
"Of how much greater faith and salutary fear are they who . . . confess their sins to the priests of God in a straightforward manner and in sorrow, making an open declaration of conscience. . . . I beseech you, brethren, let everyone who has sinned confess his sin while he is still in this world, while his confession is still admissible, while the satisfaction and remission made through the priests are still pleasing before the Lord" (ibid., 28).
"[S]inners may do penance for a set time, and according to the rules of discipline come to public confession, and by imposition of the hand of the bishop and clergy receive the right of Communion. [But now some] with their time [of penance] still unfulfilled . . . they are admitted to Communion, and their name is presented; and while the penitence is not yet performed, confession is not yet made, the hands of the bishop and clergy are not yet laid upon them, the Eucharist is given to them; although it is written, ‘Whosoever shall eat the bread and drink the cup of the Lord unworthily, shall be guilty of the body and blood of the Lord’ [1 Cor. 11:27]" (Letters 9:2 [A.D. 253]).
"And do not think, dearest brother, that either the courage of the brethren will be lessened, or that martyrdoms will fail for this cause, that penance is relaxed to the lapsed, and that the hope of peace [i.e., absolution] is offered to the penitent. . . . For to adulterers even a time of repentance is granted by us, and peace is given" (ibid., 51:20).
"But I wonder that some are so obstinate as to think that repentance is not to be granted to the lapsed, or to suppose that pardon is to be denied to the penitent, when it is written, ‘Remember whence thou art fallen, and repent, and do the first works’ [Rev. 2:5], which certainly is said to him who evidently has fallen, and whom the Lord exhorts to rise up again by his deeds [of penance], because it is written, ‘Alms deliver from death’ [Tob. 12:9]" (ibid., 51:22).
Aphraahat the Persian Sage
"You [priests], then, who are disciples of our illustrious physician [Christ], you ought not deny a curative to those in need of healing. And if anyone uncovers his wound before you, give him the remedy of repentance. And he that is ashamed to make known his weakness, encourage him so that he will not hide it from you. And when he has revealed it to you, do not make it public, lest because of it the innocent might be reckoned as guilty by our enemies and by those who hate us" (Treatises 7:3 [A.D. 340]).
Basil the Great
"It is necessary to confess our sins to those to whom the dispensation of God’s mysteries is entrusted. Those doing penance of old are found to have done it before the saints. It is written in the Gospel that they confessed their sins to John the Baptist [Matt. 3:6], but in Acts [19:18] they confessed to the apostles" (Rules Briefly Treated 288 [A.D. 374]).