The Rambam (Rabbi Moshe Maimonides, Spain-Egypt, 1135-1204), in the beginning of Hilchot Teshuba (listen to audio recording for precise citation), establishes that anytime a person transgresses a Torah law, he is required to verbally confess his sin. He emphasizes that this applies to all Biblical commands, whether one neglected a Misvat Aseh (affirmative command) or violated a Misvat Lo Taaseh (prohibition). Furthermore, the Rambam adds, one must verbally confess even for sins transgressed inadvertently.
The Rambam also presents here the text of the confession: "Please, God: I have sinned, transgressed and betrayed before You, and I did such-and-such. I hereby regret and feel ashamed over what Ive done, and I will never repeat this thing." Although this is the basic confessional text, the Rambam writes that it is admirable for a sinner to speak in greater length about the remorse he feels for the sin or sins he transgressed.
The Rambam adds that atonement is not possible without confession. In ancient times, when sinners would bring sacrifices as a means of atonement, the sacrifice earned atonement only in conjunction with Teshuba (repentance) and confession. The sinner would declare confession over his sacrifice in order to earn forgiveness. Likewise, when Bet Din would administer corporal punishment, the sinner had to verbally confess in order to earn atonement through the lashes. The same is true of violators of capital crimes such as Shabbat desecration who would be put to death by the Bet Din. They, too, would have to confess so that their sin would be atoned through their death. The Rambam adds that confession is required even for interpersonal offenses. If a person caused his fellow physical harm or damaged his property, his sin is not atoned through monetary compensation alone; he must also confess his sin to God and commit never to repeat the act.
In short, no sin can ever be forgiven without verbal confession and a commitment not to repeat the sin.
