via Lma’an Yishme’u
If a person wants to be forgiven on Yom Kippur, does he first have to do teshuvah? (repentance and returning to G-d) This question is the subject of a machlokes (disagreement) in the Gemara. Rebbi holds that even if one does not do teshuvah, the essence of the day of Yom Kippur - the mere fact that the day has passed - grants atonement, but the Chachomim (sages) say that Yom Kippur atones only for those who do teshuvah. The Halachah (Jewish law) follows the view of the Chachomim.
The (Lubavitcher) Rebbe explains that everyone agrees that the atonement of Yom Kippur is brought about by the kedushah (holiness) of the day; it is only that to attain that kedushah one is required to do teshuvah.
(שבועות ג' ע"ב, לקו"ש ח"ד ע' 1149)