Objection 1. It would seem that not only the priest is bound by the seal of confession. For sometimes a priest hears a confession through an interpreter, if there be an urgent reason for so doing. But it seems that the interpreter is bound to keep the confession secret. Therefore one who is not a priest knows something under the seal of confession.
Objection 2. Further, it is possible sometimes in cases of urgency for a layman to hear a confession. But he is bound to secrecy with regard to those sins, since they are told to him as to God. Therefore not only the priest is bound by the seal of confession.
Objection 3. Further, it may happen that a man pretends to be a priest, so that by this deceit he may know what is on another's conscience: and it would seem that he also sins if he divulges the confession. Therefore not only the priest is bound by the seal of confession.
On the contrary, A priest alone is the minister of this sacrament. But the seal of confession is connected with this sacrament. Therefore the priest alone is bound by the seal of confession.
Further, the reason why a man is bound to keep secret what he hears in confession, is because he knows them, not as man but as God knows them. But the priest alone is God's minister. Therefore he alone is bound to secrecy.
I answer that, The seal of confession affects the priest as minister of this sacrament: which seal is nothing else than the obligation of keeping the confession secret, even as the key is the power of absolving. Yet, as one who is not a priest, in a particular case has a kind of share in the act of the keys, when he hears a confession in a case of urgency, so also does he have a certain share in the act of the seal of confession, and is bound to secrecy, though, properly speaking, he is not bound by the seal of confession.
This suffices for the Replies to the Objections.
Source: Summa Theologica (New Advent)