Can a priest, with the penitent's permission, reveal to another person a sin which he knows under the seal of confession? Aquinas takes up this question in his Commentary on the Sentences of Lombard (In IV Sent., distinction 21, q. 3, a. 2 — translation of this article in the Supplement to the Summa, q. 11, a. 4).
He explains, there are two general reasons binding a priest to keep secret sins he has heard as a confessor, within confession. First, and most importantly, because in hearing a person's confession, the priest acts in behalf of God, and so knows the sins only as God's minister [or: knows the sin only as God does]. Secondly, in order to avoid scandal, that persons become unwilling or less willing to go to confession because they feel they cannot be confident that the priest will preserve the secrecy of the confession.
Aquinas goes on to say that the penitent can bring it about that what the priest previously knew only "as God" (or as God's minister), he know also "as man", and the penitent does this when he gives him permission to speak [to another person]. (quod facit dum eum licentiat ad dicendum). Consequently, if the priest then speaks to others, he does not break the seal of confession. But scandal could still be possible, if someone hears that the priest told what he heard in confession, without hearing that the penitent gave him permission to tell, and so the priest must take care to avoid scandal.
The interpretive crux of this article is, what does Aquinas means when he says that the penitent makes the priest know as man what he formerly knew as God "when he gives him permission to speak"? Commentators offer two quite different interpretations of the statement. According to one interpretation, when the penitent gives the priest permission to speak, he tells him again, now outside of confessor, the sin he wishes him to tell another person. According to the other, precisely by giving the priest permission to tell another person what the penitent told him in confession, he thereby enables the priest to know as a man what he formerly knew only as God's minister, and thereby to speak based on that knowledge outside of the sacramental context.
The principal argument in favor of the first interpretation is basically the following: Outside of confession, the priest cannot use the knowledge which he has from confession. Consequently, if the penitent tells him, "you can tell people what I told you last year in confession," the priest cannot use any knowledge of the confession to know what it is the penitent is giving him permission to tell. Even if he tells him some detail, "you can tell people what I told you in confession last year when I confessed being the one who burned that house down," the priest cannot use any other knowledge of the confession to know in more detail what the penitent was permitting him to tell.
Against this first interpretation, and in favor of the second interpretation, are these considerations: first, if the priest, even with the penitent's permission, cannot use the knowledge he has from confession, but has to reacquire it, then he can under no circumstances ever say anything about the confession, no matter how much the penitent gives him permission, and no matter how much the penitent retells outside of confession what he previously told him in confession. For on this hypothesis, if the penitent tells him "you can tell people what I told you in confession," and the penitent says "I told you X and Y", the priest would still be unable to use the knowledge he has from the confession to verify what the penitent is telling him. Thus he would only be able to say "he told me X and Y," or "he told me that he confessed X and Y," but not "he confessed X and Y". But for the priest to reveal what someone tells him outside of confession, or for a priest to say that someone told him that he confessed one thing or another, is in no way to reveal what he knew through confession. Thus the assertion that a priest, with the permission of the penitent, can reveal to another a sin that he knows under the seal of confession, would be meaningless. To be more precise, it would be only incidentally true — that which he happens to know under the seal of confession, he can reveal to another if he also knows it through another means, and the person who tells him in that other context gives him permission to pass it on.
Following up on this point, Aquinas in the immediately following article (both in the original text, the Commentary on the Sentences, and in the compiled Supplement to the Summa), asks whether a man may reveal that which he knows through confession and through some other source besides, and says that he can. According to the first interpretation of the article we are considering, it would be merely a particular case of this more general point, so it would be strange that Aquinas does not allude to this.
Finally, Aquinas certainly does not understand a priest's knowing what he heard in confession "as God" as excluding all use of that knowledge, but as excluding all use that would reveal a sin — as God covers sins that are submitted to him in penance, so must the priest, as God's minister, conceal sins submitted to him in penance. (In IV Sent. dist. 21, q. 3, a. 1, qa. 1, or supplement, q. 11, a. 1). Indeed, Aquinas opines that an abbot who learns in confession of a prior's sin that would make it disastrous for him to remain in office, he can relieve him of the office of prior on some other pretext, and thus avoid all suspicion of divulging the confession. (Note that it is now forbidden by canon law for those in authority to make any use for external governance of knowledge about sins received in confession at any time [CIC 984 § 2].)