Objection 1. It would seem that confession is not an act of faith. For the same act does not belong to different virtues. Now confession belongs to penance of which it is a part. Therefore it is not an act of faith.
Objection 2. Further, man is sometimes deterred by fear or some kind of confusion, from confessing his faith: wherefore the Apostle (Ephesians 6:19) asks for prayers that it may be granted him "with confidence, to make known the mystery of the gospel." Now it belongs to fortitude, which moderates daring and fear, not to be deterred from doing good on account of confusion or fear. Therefore it seems that confession is not an act of faith, but rather of fortitude or constancy.
Objection 3. Further, just as the ardor of faith makes one confess one's faith outwardly, so does it make one do other external good works, for it is written (Galatians 5:6) that "faith... worketh by charity." But other external works are not reckoned acts of faith. Therefore neither is confession an act of faith.
On the contrary, A gloss explains the words of 2 Thessalonians 1:11, "and the work of faith in power" as referring to "confession which is a work proper to faith."
I answer that, Outward actions belong properly to the virtue to whose end they are specifically referred: thus fasting is referred specifically to the end of abstinence, which is to tame the flesh, and consequently it is an act of abstinence.
Now confession of those things that are of faith is referred specifically as to its end, to that which concerns faith, according to 2 Corinthians 4:13: "Having the same spirit of faith... we believe, and therefore we speak also." For the outward utterance is intended to signify the inward thought. Wherefore, just as the inward thought of matters of faith is properly an act of faith, so too is the outward confession of them.
Reply to Objection 1. A threefold confession is commended by the Scriptures. One is the confession of matters of faith, and this is a proper act of faith, since it is referred to the end of faith as stated above. Another is the confession of thanksgiving or praise, and this is an act of "latria," for its purpose is to give outward honor to God, which is the end of "latria." The third is the confession of sins, which is ordained to the blotting out of sins, which is the end of penance, to which virtue it therefore belongs.
Reply to Objection 2. That which removes an obstacle is not a direct, but an indirect, cause, as the Philosopher proves (Phys. viii, 4). Hence fortitude which removes an obstacle to the confession of faith, viz. fear or shame, is not the proper and direct cause of confession, but an indirect cause so to speak.
Reply to Objection 3. Inward faith, with the aid of charity, causes all outward acts of virtue, by means of the other virtues, commanding, but not eliciting them; whereas it produces the act of confession as its proper act, without the help of any other virtue.
Source: Summa Theologica (New Advent)