Objection 1. It would seem that confession cannot be lacking in form. For it is written (Sirach 17:26): "Praise [confession] perisheth from the dead as nothing." But a man without charity is dead, because charity is the life of the soul. Therefore there can be no confession without charity.
Objection 2. Further, confession is condivided with contrition and satisfaction. But contrition and satisfaction are impossible without charity. Therefore confession is also impossible without charity.
Objection 3. Further, it is necessary in confession that the word should agree with the thought for the very name of confession requires this. Now if a man confess while remaining attached to sin, his word is not in accord with his thought, since in his heart he holds to sin, while he condemns it with his lips. Therefore such a man does not confess.
On the contrary, Every man is bound to confess his mortal sins. Now if a man in mortal sin has confessed once, he is not bound to confess the same sins again, because, as no man knows himself to have charity, no man would know of him that he had confessed. Therefore it is not necessary that confession should be quickened by charity.
I answer that, Confession is an act of virtue, and is part of a sacrament. In so far as it is an act of virtue, it has the property of being meritorious, and thus is of no avail without charity, which is the principle of merit. But in so far as it is part of a sacrament, it subordinates the penitent to the priest who has the keys of the Church, and who by means of the confession knows the conscience of the person confessing. In this way it is possible for confession to be in one who is not contrite, for he can make his sins known to the priest, and subject himself to the keys of the Church: and though he does not receive the fruit of absolution then, yet he will begin to receive it, when he is sincerely contrite, as happens in the other sacraments: wherefore he is not bound to repeat his confession, but to confess his lack of sincerity.
Reply to Objection 1. These words must be understood as referring to the receiving of the fruit of confession, which none can receive who is not in the state of charity.
Reply to Objection 2. Contrition and satisfaction are offered to God: but confession is made to man: hence it is essential to contrition and satisfaction, but not to confession, that man should be united to God by charity.
Reply to Objection 3. He who declares the sins which he has, speaks the truth; and thus his thought agrees with his lips or words, as to the substance of confession, though it is discordant with the purpose of confession.
Source: Summa Theologica (New Advent)