Objection 1. It would seem that it is lawful for a man to confess a sin which he has not committed. For, as Gregory says (Regist. xii), "it is the mark of a good conscience to acknowledge a fault where there is none." Therefore it is the mark of a good conscience to accuse oneself of those sins which one has not committed.
Objection 2. Further, by humility a man deems himself worse than another, who is known to be a sinner, and in this he is to be praised. But it is lawful for a man to confess himself to be what he thinks he is. Therefore it is lawful to confess having committed a more grievous sin than one has.
Objection 3. Further, sometimes one doubts about a sin, whether it be mortal or venial, in which case, seemingly, one ought to confess it as mortal. Therefore a person must sometimes confess a sin which he has not committed.
Objection 4. Further, satisfaction originates from confession. But a man can do satisfaction for a sin which he has not committed. Therefore he can also confess a sin which he has not done.
On the contrary, Whosoever says he has done what he did not, tells an untruth. But no one ought to tell an untruth in confession, since every untruth is a sin. Therefore no one should confess a sin which he has not committed.
Further, in the public court of justice, no one should be accused of a crime which cannot be proved by means of proper witnesses. Now the witness, in the tribunal of Penance, is the conscience. Therefore a man ought not to accuse himself of a sin which is not on his conscience.
I answer that, The penitent should, by his confession, make his state known to his confessor. Now he who tells the priest something other than what he has on his conscience, whether it be good or evil, does not make his state known to the priest, but hides it; wherefore his confession is unavailing: and in order for it to be effective his words must agree with his thoughts, so that his words accuse him only of what is on his conscience.
Reply to Objection 1. To acknowledge a fault where there is none, may be understood in two ways: first, as referring to the substance of the act, and then it is untrue; for it is a mark, not of a good, but of an erroneous conscience, to acknowledge having done what one has not done. Secondly, as referring to the circumstances of the act, and thus the saying of Gregory is true, because a just man fears lest, in any act which is good in itself, there should be any defect on his part. thus it is written (Job 9:28): "I feared all my works." Wherefore it is also the mark of a good conscience that a man should accuse himself in words of this fear which he holds in his thoughts.
From this may be gathered the Reply to the Second Objection, since a just man, who is truly humble, deems himself worse not as though he had committed an act generically worse, but because he fears lest in those things which he seems to do well, he may by pride sin more grievously.
Reply to Objection 3. When a man doubts whether a certain sin be mortal, he is bound to confess it, so long as he remains in doubt, because he sins mortally by committing or omitting anything, while doubting of its being a mortal sin, and thus leaving the matter to chance; and, moreover, he courts danger, if he neglect to confess that which he doubts may be a mortal sin. He should not, however, affirm that it was a mortal sin, but speak doubtfully, leaving the verdict to the priest, whose business it is to discern between what is leprosy and what is not.
Reply to Objection 4. A man does not commit a falsehood by making satisfaction for a sin which he did not commit, as when anyone confesses a sin which he thinks he has not committed. And if he mentions a sin that he has not committed, believing that he has, he does not lie; wherefore he does not sin, provided his confession thereof tally with his conscience.
Source: Summa Theologica (New Advent)