Objection 1. It would seem that confession is not necessary for salvation. For the sacrament of Penance is ordained for the sake of the remission of sin. But sin is sufficiently remitted by the infusion of grace. Therefore confession is not necessary in order to do penance for one's sins.
Objection 2. Further, we read of some being forgiven their sins without confession, e.g. Peter, Magdalen and Paul. But the grace that remits sins is not less efficacious now than it was then. Therefore neither is it necessary for salvation now that man should confess.
Objection 3. Further, a sin which is contracted from another, should receive its remedy from another. Therefore actual sin, which a man has committed through his own act, must take its remedy from the man himself. Now Penance is ordained against such sins. Therefore confession is not necessary for salvation.
Objection 4. Further, confession is necessary for a judicial sentence, in order that punishment may be inflicted in proportion to the offense. Now a man is able to inflict on himself a greater punishment than even that which might be inflicted on him by another. Therefore it seems that confession is not necessary for salvation.
On the contrary, Boethius says (De Consol. i): "If you want the physician to be of assistance to you, you must make your disease known to him." But it is necessary for salvation that man should take medicine for his sins. Therefore it is necessary for salvation that man should make his disease known by means of confession.
Further, in a civil court the judge is distinct from the accused. Therefore the sinner who is the accused ought not to be his own judge, but should be judged by another and consequently ought to confess to him.
I answer that, Christ's Passion, without whose power, neither original nor actual sin is remitted, produces its effect in us through the reception of the sacraments which derive their efficacy from it. Wherefore for the remission of both actual and original sin, a sacrament of the Church is necessary, received either actually, or at least in desire, when a man fails to receive the sacrament actually, through an unavoidable obstacle, and not through contempt. Consequently those sacraments which are ordained as remedies for sin which is incompatible with salvation, are necessary for salvation: and so just as Baptism, whereby original sin is blotted out, is necessary for salvation, so also is the sacrament of Penance. And just as a man through asking to be baptized, submits to the ministers of the Church, to whom the dispensation of that sacrament belongs, even so, by confessing his sin, a man submits to a minister of the Church, that, through the sacrament of Penance dispensed by him, he may receive the pardon of his sins: nor can the minister apply a fitting remedy, unless he be acquainted with the sin, which knowledge he acquires through the penitent's confession. Wherefore confession is necessary for the salvation of a man who has fallen into a mortal actual sin.
Reply to Objection 1. The infusion of grace suffices for the remission of sin; but after the sin has been forgiven, the sinner still owes a debt of temporal punishment. Moreover, the sacraments of grace are ordained in order that man may receive the infusion of grace, and before he receives them, either actually or in his intention, he does not receive grace. This is evident in the case of Baptism, and applies to Penance likewise. Again, the penitent expiates his temporal punishment by undergoing the shame of confession, by the power of the keys to which he submits, and by the enjoined satisfaction which the priest moderates according to the kind of sins made known to him in confession. Nevertheless the fact that confession is necessary for salvation is not due to its conducing to the satisfaction for sins, because this punishment to which one remains bound after the remission of sin, is temporal, wherefore the way of salvation remains open, without such punishment being expiated in this life: but it is due to its conducing to the remission of sin, as explained above.
Reply to Objection 2. Although we do not read that they confessed, it may be that they did; for many things were done which were not recorded in writing. Moreover Christ has the power of excellence in the sacraments; so that He could bestow the reality of the sacrament without using the things which belong to the sacrament.
Reply to Objection 3. The sin that is contracted from another, viz. original sin, can be remedied by an entirely extrinsic cause, as in the case of infants: whereas actual sin, which a man commits of himself, cannot be expiated, without some operation on the part of the sinner. Nevertheless man is not sufficient to expiate his sin by himself, though he was sufficient to sin by himself, because sin is finite on the part of the thing to which it turns, in which respect the sinner returns to self; while, on the part of the aversion, sin derives infinity, in which respect the remission of sin must needs begin from someone else, because "that which is last in order of generation is first in the order of intention" (Ethic. iii). Consequently actual sin also must needs take its remedy from another.
Reply to Objection 4. Satisfaction would not suffice for the expiation of sin's punishment, by reason of the severity of the punishment which is enjoined in satisfaction, but it does suffice as being a part of the sacrament having the sacramental power; wherefore it ought to be imposed by the dispensers of the sacraments, and consequently confession is necessary.
Source: Summa Theologica (New Advent)