Objection 1. It would seem that it is never lawful to confess to another than a priest. For confession is a sacramental accusation, as appears from the definition given above (Question 7, Article 1). But the dispensing of a sacrament belongs to none but the minister of a sacrament. Since then the proper minister of Penance is a priest, it seems that confession should be made to no one else.
Objection 2. Further, in every court of justice confession is ordained to the sentence. Now in a disputed case the sentence is void if pronounced by another than the proper judge; so that confession should be made to none but a judge. But, in the court of conscience, the judge is none but a priest, who has the power of binding and loosing. Therefore confession should be made to no one else.
Objection 3. Further, in the case of Baptism, since anyone can baptize, if a layman has baptized, even without necessity, the Baptism should not be repeated by a priest. But if anyone confess to a layman in a case of necessity, he is bound to repeat his confession to a priest, when the cause for urgency has passed. Therefore confession should not be made to a layman in a case of necessity.
On the contrary, is the authority of the text (Sent. iv, D, 17).
I answer that, Just as Baptism is a necessary sacrament, so is Penance. And Baptism, through being a necessary sacrament has a twofold minister: one whose duty it is to baptize, in virtue of his office, viz. the priest, and another, to whom the conferring of Baptism is committed, in a case of necessity. In like manner the minister of Penance, to whom, in virtue of his office, confession should be made, is a priest; but in a case of necessity even a layman may take the place of a priest, and hear a person's confession.
Reply to Objection 1. In the sacrament of Penance there is not only something on the part of the minister, viz. the absolution and imposition of satisfaction, but also something on the part of the recipient, which is also essential to the sacrament, viz. contrition and confession. Now satisfaction originates from the minister in so far as he enjoins it, and from the penitent who fulfills it; and, for the fulness of the sacrament, both these things should concur when possible. But when there is reason for urgency, the penitent should fulfill his own part, by being contrite and confessing to whom he can; and although this person cannot perfect the sacrament, so as to fulfill the part of the priest by giving absolution, yet this defect is supplied by the High Priest. Nevertheless confession made to a layman, through lack* of a priest, is quasi-sacramental, although it is not a perfect sacrament, on account of the absence of the part which belongs to the priest. [Here and in the Reply to Objection 2 the Leonine edition reads "through desire for a priest".]
Reply to Objection 2. Although a layman is not the judge of the person who confesses to him, yet, on account of the urgency, he does take the place of a judge over him, absolutely speaking, in so far as the penitent submits to him, through lack of a priest.
Reply to Objection 3. By means of the sacraments man must needs be reconciled not only to God, but also to the Church. Now he cannot be reconciled to the Church, unless the hallowing of the Church reach him. In Baptism the hallowing of the Church reaches a man through the element itself applied externally, which is sanctified by "the word of life" (Ephesians 5:26), by whomsoever it is conferred: and so when once a man has been baptized, no matter by whom, he must not be baptized again. On the other hand, in Penance the hallowing of the Church reaches man by the minister alone, because in that sacrament there is no bodily element applied externally, through the hallowing of which grace may be conferred. Consequently although the man who, in a case of necessity, has confessed to a layman, has received forgiveness from God, for the reason that he fulfilled, so far as he could, the purpose which he conceived in accordance with God's command, he is not yet reconciled to the Church, so as to be admitted to the sacraments, unless he first be absolved by a priest, even as he who has received the Baptism of desire, is not admitted to the Eucharist. Wherefore he must confess again to a priest, as soon as there is one at hand, and the more so since, as stated above (ad 1), the sacrament of Penance was not perfected, and so it needs yet to be perfected, in order that by receiving the sacrament, the penitent may receive a more plentiful effect, and that he may fulfill the commandment about receiving the sacrament of Penance.
Source: Summa Theologica (New Advent)