Objection 1. It would seem that the priest is not bound in every case to hide the sins which he knows under the seal of confession. For, as Bernard says (De Proecep. et Dispens. ii), "that which is instituted for the sake of charity does not militate against charity." Now the secret of confession would militate against charity in certain cases: for instance, if a man knew through confession that a certain man was a heretic, whom he cannot persuade to desist from misleading the people; or, in like manner, if a man knew, through confession, that certain people who wish to marry are related to one another. Therefore such ought to reveal what they know through confession.
Objection 2. Further, that which is obligatory solely on account of a precept of the Church need not be observed, if the commandment be changed to the contrary. Now the secret of confession was introduced solely by a precept of the Church. If therefore the Church were to prescribe that anyone who knows anything about such and such a sin must make it known, a man that had such knowledge through confession would be bound to speak.
Objection 3. Further, a man is bound to safeguard his conscience rather than the good name of another, because there is order in charity. Now it happens sometimes that a man by hiding a sin injures his own conscience--for instance, if he be called upon to give witness of a sin of which he has knowledge through confession, and is forced to swear to tell the truth--or when an abbot knows through confession the sin of a prior who is subject to him, which sin would be an occasion of ruin to the latter, if he suffers him to retain his priorship, wherefore he is bound to deprive him of the dignity of his pastoral charge, and yet in depriving him he seem to divulge the secret of confession. Therefore it seems that in certain cases it is lawful to reveal a confession.
Objection 4. Further, it is possible for a priest through hearing a man's confession to be conscious that the latter is unworthy of ecclesiastical preferment. Now everyone is bound to prevent the promotion of the unworthy, if it is his business. Since then by raising an objection he seems to raise a suspicion of sin, and so to reveal the confession somewhat, it seems that it is necessary sometimes to divulge a confession.
On the contrary, The Decretal says (De Poenit. et Remiss., Cap. Omnis utriusque): "Let the priest beware lest he betray the sinner, by word, or sign, or in any other way whatever."
Further, the priest should conform himself to God, Whose minister he is. But God does not reveal the sins which are made known to Him in confession, but hides them. Neither, therefore, should the priest reveal them.
I answer that, Those things which are done outwardly in the sacraments are the signs of what takes place inwardly: wherefore confession, whereby a man subjects himself to a priest, is a sign of the inward submission, whereby one submits to God. Now God hides the sins of those who submit to Him by Penance; wherefore this also should be signified in the sacrament of Penance, and consequently the sacrament demands that the confession should remain hidden, and he who divulges a confession sins by violating the sacrament. Besides this there are other advantages in this secrecy, because thereby men are more attracted to confession, and confess their sins with greater simplicity.
Reply to Objection 1. Some say that the priest is not bound by the seal of confession to hide other sins than those in respect of which the penitent promises amendment; otherwise he may reveal them to one who can be a help and not a hindrance. But this opinion seems erroneous, since it is contrary to the truth of the sacrament; for just as, though the person baptized be insincere, yet his Baptism is a sacrament, and there is no change in the essentials of the sacrament on that account, so confession does not cease to be sacramental although he that confesses, does not purpose amendment. Therefore, this notwithstanding, it must be held secret; nor does the seal of confession militate against charity on that account, because charity does not require a man to find a remedy for a sin which he knows not: and that which is known in confession, is, as it were, unknown, since a man knows it, not as man, but as God knows it. Nevertheless in the cases quoted one should apply some kind of remedy, so far as this can be done without divulging the confession, e.g. by admonishing the penitent, and by watching over the others lest they be corrupted by heresy. He can also tell the prelate to watch over his flock with great care, yet so as by neither word nor sign to betray the penitent.
Reply to Objection 2. The precept concerning the secret of confession follows from the sacrament itself. Wherefore just as the obligation of making a sacramental confession is of Divine law, so that no human dispensation or command can absolve one therefrom, even so, no man can be forced or permitted by another man to divulge the secret of confession. Consequently if he be commanded under pain of excommunication to be incurred "ipso facto," to say whether he knows anything about such and such a sin, he ought not to say it, because he should assume that the intention of the person in commanding him thus, was that he should say what he knew as man. And even if he were expressly interrogated about a confession, he ought to say nothing, nor would he incur the excommunication, for he is not subject to his superior, save as a man, and he knows this not as a man, but as God knows it.
Reply to Objection 3. A man is not called upon to witness except as a man, wherefore without wronging his conscience he can swear that he knows not, what he knows only as God knows it. In like manner a superior can, without wronging his conscience, leave a sin unpunished which he knows only as God knows it, or he may forbear to apply a remedy, since he is not bound to apply a remedy, except according as it comes to his knowledge. Wherefore with regard to matters which come to his knowledge in the tribunal of Penance, he should apply the remedy, as far as he can, in the same court: thus as to the case in point, the abbot should advise the prior to resign his office, and if the latter refuse, he can absolve him from the priorship on some other occasion, yet so as to avoid all suspicion of divulging the confession.
Reply to Objection 4. A man is rendered unworthy of ecclesiastical preferment, by many other causes besides sin, for instance, by lack of knowledge, age, or the like: so that by raising an objection one does not raise a suspicion of crime or divulge the secret of confession.
Source: Summa Theologica (New Advent)