The foundational talmudic debate over whether itzumo shel Yom Kippur atones independently of teshuvah is recorded in Yoma 85b, where Rabbi Yehuda HaNasi holds that for virtually all transgressions — whether or not one repented — Yom Kippur atones, while the Gemara notes that the mishna itself implies teshuvah is required, setting up the classical machlokes between Rabbi and the Sages.
The Kessef Mishneh (Mishneh Torah, Repentance 1:3) makes clear that the halacha does not follow Rabbi, since the Sages dispute him; accordingly, the Rambam (Mishneh Torah, Repentance 1:3) rules that itzumo shel Yom Kippur atones only for those who repent — "לשבים" — but not for those who do not.
The standard three-tiered structure of atonement recorded in Yoma 86a — where violation of a positive commandment is forgiven immediately upon repentance, a prohibition requires repentance plus Yom Kippur's atonement, and sins punishable by karet or death require repentance together with both Yom Kippur and yissurim — is the framework within which itzumo shel Yom Kippur finds its place, always paired with teshuvah according to the Sages.
Pirkei DeRabbi Eliezer 46:7 amplifies the cosmic scope of Yom Kippur's atonement, reading the pasuk "מִכֹּל חַטֹּאתֵיכֶם" to mean that Yom Kippur atones for both light and grave sins, and that without it the world could not endure — a reading that underscores how the day itself carries inherent atoning power, even as the halachic question of whether that power operates without teshuvah remains contested.
Peninei Halakhah, Days of Awe 6:8 articulates Rabbi's position as a theological claim about the eternal covenant between God and Israel: because the atonement is a divine decree rooted in that unbreakable bond, even one who declares he does not want Yom Kippur to atone for him is atoned against his will — as the Yerushalmi (Shevuot 1:6) puts it, no person can tell the King not to reign over him.
Peninei Halakhah, Days of Awe 6:9 presents the position of the Sages as a middle path: while Yom Kippur does atone for the collective of Israel, it does not save the individual from the strict judgment he deserves unless he has expressed his inner will to be good — concretely, by fasting, refraining from labor, and praying, thereby revealing that his deeper self desires to be righteous, even without complete teshuvah in the fullest sense.
Resisei Layla 54:4 adopts an approach close to Rabbi's, arguing that Yom Kippur's core atonement belongs to Israel alone and that even for those who have not formally repented the atonement is valid, because God knows that Israel's true inner will is to do His will — it is only the yetzer hara and the exile that cause their sins, so the sins are likened to thorns surrounding a myrtle whose essence remains pure.
The Tanya, Iggeret HaTeshuvah (Part III, ch. 1) cites the same three-tiered baraita and launches from it an extended Hasidic analysis of how Yom Kippur atones for violations of prohibitions when accompanied by teshuvah, while the Tanya, Iggeret HaTeshuvah (Part III, ch. 11) notes that Yom Kippur atones for more severe transgressions that the daily tamid — and by extension daily prayer — cannot reach.
The Rambam (Mishneh Torah, Repentance 2:9) and the Shulchan Arukh (Orach Chayim 606:1) together establish an essential boundary on the reach of itzumo shel Yom Kippur regardless of which side of the machlokes one follows: Yom Kippur, with or without teshuvah, atones only for sins between a person and God — sins between a person and his fellow are not forgiven until the wronged party is appeased.