Yom Kippur atones only for the repentant (shavim) who believe in its atonement. But one who scoffs at it and thinks, "what does Yom Kippur do for me?" does not gain atonement on Yom Kippur (Rama, Orach Chaim 607:6).
Yom Kippur atones with teshuva; without teshuva it does not atone. Rebbe says it does atone without teshuva (Yoma 85b).
What is teshuva? The sinner must cease sinning, regret his sin, and resolve not to commit the sin again (Rambam, Hil. Teshuva 2:2).
At first glance, there is absolutely no atonement on Yom Kippur unless a person does teshuva; but, when accompanying teshuva, Yom Kippur completes atonement for the violation of mitvos lo-ta'asei - negative commandments (Yoma 86a, Rambam 1:4). The Rama, however, implies that as long as one believes in the effectiveness of Yom Kippur it atones, even in the absence of teshuva. The Rama's source is the Rambam (Hil. Shegagos 3:10, based on Krisus 7a), but the Rambam himself states that the essence of Yom Kippur atones for the repentant (shavim) (Hil. Teshuva 1:3)! The Kesef Mishna, in his explanation of the Rambam, cites the aforementioned dispute between Rebbe and the Rabanan. This citation implies a need for full teshuva according to the Rambam, not merely a belief in the effectiveness of Yom Kippur.
Perhaps the understanding of the Rama and Rambam is that only a full atonement of Yom Kippur requires a full teshuva. However, there is a partial atonement on Yom Kippur for anyone who believes in it. Such a person is in the class of shavim, even though he does not regret and cease his sin, and does not resolve not to repeat it.
The concept of a partial atonement on Yom Kippur is found according to Rebbe for one who is unrepentant and eats on Yom Kippur (Tosafos Yeshanim, Yoma 85b). Yom Kippur serves to avoid kares, but teshuva is needed to achieve full atonement.
A similar concept exists according to the Rabanan for one who is repentant. If he believes in the effectiveness of Yom Kippur, it provides partial atonement. Full atonement, however, requires teshuva.
Yom Kippur is the day on which Am Yisrael was forgiven for the sin of the golden calf. For this reason it was established as a day of atonement (Rashi Devarim 9:18).
The origin of this sin was stiff-neckedness (am k'shei oref- Devarim 9:6). It was this attribute that triggered Hashem's threat to destroy us(Shemos 32:9,10). A stiff neck contains, metaphorically, a metal rod which makes a person unable to turn his neck. This prevents him from turning to face a rav whose compelling message deters sin. This trait eliminates any hope for teshuva (Sforno, Devarim 9:6-8).
The ideal teshuva, based on love of Hashem, returns a person all the way to (ad) Hashem and His throne (Hoshea 14:1, Yoma 86a). The next pasuk exhorts teshuva to (el) Hashem, which may refer to one who does not reach all the way to Hashem, but returns towards Hashem.
On Yom Kippur we need only turn around to face (lifnei) Hashem in order to gain a measure of atonement and purity. Belief in Yom Kippur's effectiveness is, in itself, such a gesture, which makes a person repentant.
Turning around is the opposite of stiff-neckedness. Since the origin of Yom Kippur is atonement for the stiff-neckedness of the cheit ha-egel, even a mere turning around towards Hashem achieves partial atonement.
At Ne'ila we proclaim that we, and our eyes, look to Hashem. Our ancestors faced East and worhsipped the sun. In the Beis Hamikdash the kohanim turned around to the West, to the mikdash, and said we and our eyes look to Hashem (Sukkah 51b). Turning one's face to Hashem at Ne'ila confers a status of repentant and grants a measure of atonement. The full atonement of Yom Kippur requires teshuva - regret, sincere confession and acceptance not to sin in the future.
All of the nation [has sinned] unintentionally (bishgaga Bamidbar 15:26). The literal context of this pasuk atones for those who sin based on an error of the Beis Din. The recital of the pasuk three times after kol nidrei clearly applies it to all the sins of Am Yisrael. The reality of intentional sin makes this generalization quite problematic.
The expressions chatasi and avisi refer to unintentional and intentional sins, respectively (Yoma 36b). Yet the Rambam (Teshuva 1:1) seems to require both expressions in the confession of even a single sin.
Rav Soloveitchik zt'l explained that on can never be completely sure if a particular sin is unintentional. Perhaps he should have known better. Conversely, an intentional sin may be extenuated by a deficient education, a pervasive zeitgeist, difficult personal conditions or the negative influence of friends (see Al Hateshuva, p. 64).
While these mitigating factors may not technically qualify as shogeg, one cannot be certain and should therefore say chatasi even if he sinned intentionally. On Yom Kippur we invoke Hashem's mercy and characterize the sins of all Am Yisrael as sh'gaga worthy of at least partial atonement.
Indeed, Hashem forgave the sin of the golden calf on Yom Kippur based on Moshe's words (Rashi Devarim 9:18, although the pasuk's original context is the sin of the spies). The recital of this pasuk three times after Kol Nidrei (salachti k'd'varecha - Bamidbar 14:20) indicates that the key to the atonement of Yom Kippur lies in Moshe's defense of Am Yisrael's idolatry.
Moshe said before Hashem, "because of the silver and gold that you lavished upon Yisrael they made the golden calf. What should that son (who is placed in a vulnerable and tempting situation) do that he not sin?" Hashem concurred with Moshe, as it says (Hoshea 2:10) "and I lavished silver upon her, and they made [a statue of] gold to the Ba'al" (Berachos 32a). Although Moshe only advanced mitigating factors (Maharsha), Hashem forgave Am Yisrael based on his defense.
As we prepare for Yom Kippur, the following lessons emerge from our analysis of the sources.
Every person must aim for a complete teshuva, regretting and confessing his sins and resolving not to repeat them. This teshuva, which is effective all year long, combines with Yom Kippur to atone completely for negative commandments.
One who is unable to achieve this level of teshuva should not despond. Hashem looks at extenuating circumstances, especially on Yom Kippur. Turning towards Hashem and believing in the effectiveness of Yom Kippur achieve partial atonement.
Finally, in judging others, these factors must be emphasized. The importance of this idea is the very essence of the pesukim we recite after Kol Nidrei. Indeed, the opening line of the machzor, permitting us to pray together with sinners, is the preface to all of tefillah on Yom Kippur.
Our charitable attitude towards sinners reflects the indispensability of their participation in our fast (Krisus 6b). Remarkably, even today many alienated Jews fast on Yom Kippur. We should encourage and appreciate this phenomenon, even as we hold ourselves to a higher standard. May we all be worthy of atonement on this Yom Kippur, and may all of Israel be blessed with a g'mar chasima tova.