Teshuvah

The Indispensable Idea in the Torah
Repentance is one of those principles which are an indispensable element in the creed of the followers of the Torah. For it is impossible for man to be entirely free from error and sin; he either does not know the opinion which he has to choose, or he adopts a principle, not for its own merits, but in order to gratify his desire or passion. If we were convinced that we could never make our crooked ways straight, we should for ever continue in our errors, and perhaps add other sins to them since we did not see that any remedy was left to us. But the belief in the effect of repentance causes us to improve, to return to the best of the ways, and to become more perfect than we were before we sinned. For this reason many things are prescribed for the promotion of this very useful principle: e.g., confessions and sacrifices for sins committed unknowingly, and in some cases even for sins committed intentionally, and fasts, and that which is common to all cases of repentance from sin, the resolve to discontinue sinning. For that is the aim of this principle. Of all these precepts the use is obvious. (Moreh 3:36)

Yom Kippur
The object of the Fast of Atonement is evident. The Fast creates the sense of repentance: it is the same day on which the chief of all prophets came down [from Mount Sinai] with the second tablets, and announced to the people the divine pardon of their great sin: the day was therefore appointed for ever as a day devoted to repentance and true worship of God. For this reason all material enjoyment, all trouble and care for the body, are interdicted, no work may be done; the day must be spent in confession; everyone shall confess his sins and abandon them. (Moreh 3:43)