The essence of a Jews soul is free from sin-Teshuvah reveals the essence:[1]

The Chassidic teachings explain that the Jewish soul is an actual part of G-d above which contains five levels. The lowest level relates to action, the second to emotion, the third to intellect, the fourth to will, and the fifth is the essence of the soul. The first four levels, being that they are not the essence, have a weaker unity with its source, which is Hashem. In order to be fully united with Hashem, they require fulfilling the will of Hashem, which are the Mitzvos found in the Torah. Due to this, these four levels become affected by sin, becoming blemished and stained through sin, making it cause a separation in its unity with Hashem. However, the essence of the soul is absolutely one with G-d Himself, even without fulfillment of the laws of the Torah, and just as one cannot affect or blemish G-d through sin, so too this part of the soul cannot be affected by sin. On Yom Kippur, it is this part of the soul that is revealed; the essence of a Jew which is one with Hashem and immune to all obstacles threatening separation from its source. This revelation thus cleanses the other aspects of the soul, removing from it all of its blemishes caused by sin, and reuniting it with its source. However, as mentioned before, according to the final law, one is required to repent to merit this revelation. For this reason, the prayers of Yom Kippur are filled with confessions, as this is a part of Teshuvah. According to the above, Yom Kippur doesn’t mean “The day of atonement” but rather “The day is atonement,” as it is the day itself which atones.

[1] Likkutei Sichos 4 p. 1149; 27 p. 124