The sages say that repentance preceded the creation of the world, which means that repentance transports a person above and beyond the realities of the created world, with its order of time, forming, as it were, a new creation.
And since Yom Kippur is the day of Divine pardon and forgiveness, it is the revelation of a Supreme Essence that transcends the limits of the whole world.
The commentary on the verse ‘I, I alone, am He who wipes away your transgressions for My sake’ (Isaiah 43:25) places the words ‘I, I alone’ on a higher level than the ‘I’ with which the Ten Commandments begins (Exodus 20:2).
This revelation, which transcends and cuts through the boundaries of the world, is the essence of this day, and its power is defined in the words of the sages as ‘the very day itself atones’ (Yoma 87a).”
–Rabbi Adin Steinsaltz
From “Days of Awe” in A Guide To Jewish Prayer, p. 195-196, by Rabbi Adin Steinsaltz