We have three days to be completely purified, “Before Hashem you shall be purified” (Vayikra 16:30) — “Purified — Tit’haru” is the numerical value of Keter — Crown.
Rabbi Yehuda HaNasi says that the essence of the day atones, only on the condition that he accepts onto himself not to commit more transgressions.
The essence of the day atones, for every transgression, even if a person trips up on Yom Kippur, if he thinks to repent for this sin, it immediately atones for him.
Then according to this, when is he liable to karet — spiritually excision?
He’s even exempt from karet. Then only on willful sins, even he does this in the last second, so that he can no longer repent on Yom Kippur.
Only if he sinned, G-d forbid, in the last second mamash before the stars come out, the day doesn’t atone. He needs to repent and then everything is atoned for.
From the “Shevivi Or” newsletter.