Yom Kippur is a time of forgiveness and atonement, as we read, “For on this day, atonement shall be made for you to purify you of all your sins; you shall purify yourselves before the Lord” (Vayikra 16:30). Therefore, a mitzva is incumbent upon every individual to repent and to confess his sins on this day (MT, Laws of Repentance 2:7).
The process of repentance is completed when a person explicitly verbalizes a confession. As is the case with our mission in this world in general, here too, we must actualize the good intentions present in our thoughts and hearts. Explicit, verbal confession clarifies and crystalizes the thoughts and emotions accompanying repentance. Regret is deeply and keenly felt, and the penitent’s resolution to refrain from sin is reinforced. This is why the Torah commands those bringing an offering to confess their sins, as we read, “Speak to the children of Israel: When a man or woman commits any wrong toward a fellow man, thus breaking faith with the Lord, and that person realizes his guilt, he shall confess the wrong that he has done” (Bamidbar 5:6-7). Similarly, we read, “When he realizes his guilt in any of these matters, he shall confess that wherein he has sinned” (Vayikra 5:5). Just as someone offering a sin offering must confess, so too, any penitent must confess. This is the final step of his repentance (MT, Laws of Repentance 1:1).
Confession should also be practiced all year round. If one sinned unknowingly, he should say “I have sinned” (ḥatati); if he sinned knowingly, he should say “I have done wrong” (aviti); and if he sinned spitefully, he should say “I have rebelled” (pashati). By doing so, he has fulfilled the mitzva of confession, even though he has not detailed specific sins. In general, it is preferable to specify sins, although there are some situations where it is preferable not to do so (SA 607:2; MB ad loc. 5; SHT ad loc. 11; see sections 4-6 below for details).
Rambam writes, “When a person repents and turns away from his sin, he must confess before God, blessed be He” (MT, Laws of Repentance 1:1). Many understand Rambam to be saying that repentance is not a positive mitzva per se, as the necessity for it goes without saying; rather, the mitzva is vidui which is part of the process of repentance (Pri Ḥadash; Minḥat Ḥinukh; Mishpat Kohen §128). Others maintain that Rambam considers repentance to be a mitzva, whereas vidui is a necessary part of that mitzva (Mabit, Kiryat Sefer).