Objection 1. It seems that sinners who are going to be baptized are bound to confess their sins. For it is written (Matthew 3:6) that many "were baptized" by John "in the Jordan confessing their sins." But Christ's Baptism is more perfect than John's. Therefore it seems that there is yet greater reason why they who are about to receive Christ's Baptism should confess their sins.
Objection 2. Further, it is written (Proverbs 28:13): "He that hideth his sins, shall not prosper; but he that shall confess and forsake them, shall obtain mercy." Now for this is a man baptized, that he may obtain mercy for his sins. Therefore those who are going to be baptized should confess their sins.
Objection 3. Further, Penance is required before Baptism, according to Acts 2:38: "Do penance and be baptized every one of you." But confession is a part of Penance. Therefore it seems that confession of sins should take place before Baptism.
On the contrary, Confession of sins should be sorrowful: thus Augustine says (De Vera et Falsa Poenit. xiv): "All these circumstances should be taken into account and deplored." Now, as Ambrose says on Romans 11:29, "the grace of God requires neither sighs nor groans in Baptism." Therefore confession of sins should not be required of those who are going to be baptized.
I answer that, Confession of sins is twofold. One is made inwardly to God: and such confession of sins is required before Baptism: in other words, man should call his sins to mind and sorrow for them; since "he cannot begin the new life, except he repent of his former life," as Augustine says in his book on Penance (Serm. cccli). The other is the outward confession of sins, which is made to a priest; and such confession is not required before Baptism. First, because this confession, since it is directed to the person of the minister, belongs to the sacrament of Penance, which is not required before Baptism, which is the door of all the sacraments. Secondly, because the reason why a man makes outward confession to a priest, is that the priest may absolve him from his sins, and bind him to works of satisfaction, which should not be enjoined on the baptized, as stated above (Article 5). Moreover those who are being baptized do not need to be released from their sins by the keys of the Church, since all are forgiven them in Baptism. Thirdly, because the very act of confession made to a man is penal, by reason of the shame it inflicts on the one confessing: whereas no exterior punishment is enjoined on a man who is being baptized.
Therefore no special confession of sins is required of those who are being baptized; but that general confession suffices which they make when in accordance with the Church's ritual they "renounce Satan and all his works." And in this sense a gloss explains Matthew 3:6, saying that in John's Baptism "those who are going to be baptized learn that they should confess their sins and promise to amend their life."
If, however, any persons about to be baptized, wish, out of devotion, to confess their sins, their confession should be heard; not for the purpose of enjoining them to do satisfaction, but in order to instruct them in the spiritual life as a remedy against their vicious habits.
Reply to Objection 1. Sins were not forgiven in John's Baptism, which, however, was the Baptism of Penance. Consequently it was fitting that those who went to receive that Baptism, should confess their sins, so that they should receive a penance in proportion to their sins. But Christ's Baptism is without outward penance, as Ambrose says (on Romans 11:29); and therefore there is no comparison.
Reply to Objection 2. It is enough that the baptized make inward confession to God, and also an outward general confession, for them to "prosper and obtain mercy": and they need no special outward confession, as stated above.
Reply to Objection 3. Confession is a part of sacramental Penance, which is not required before Baptism, as stated above: but the inward virtue of Penance is required.
Source: Summa Theologica (New Advent)