At the end of Seder Avodat Yom haKippurim, the Torah summarizes the
goal of the day's service: "כי ביום הזה יכפר עליכם לטהר אתכם מכל
חטאתיכם לפני ה' תטהרו" - "Because on this day, he will atone for you
to purify you from all your sins before God you will be purified."

Nechama Lebowitz points out that the syntax of the פסוק is ambiguous.
It is not clear how to punctuate it, where to put the commas or
semicolons. Nechama offers three different possibilities and points
out that each choice provides a very different read and message.

The first possibility suggests that the verse is composed of 2
separate sentences: On this day he will atone for you to purify you
from your sins. Before God you will be purified. Rav Soloveichik
suggests that when one sins two things come as a consequence, one
deserves a punishment and one becomes impure. The sin doesn't only
demand retribution but also leaves its imprint on the sinner, dirtying
him, corrupting him. Thus on Yom haKippurim two things must happen.
Hashem atones for us, removes our punishment and He also purifies us,
cleansing our neshamot.

A second possible read of the verse emerges from the Mishnah in Yoma.
According to it, the verse has one main idea which is clarified in its
second half: 1) On this day he will atone for you to purify you. 2)
From all your sins before God you will be purified. The second half
of the verse explains the first -- on Yom haKippurim only your sins
before God (bein adam lamakom) are forgiven. Sins between people are
not, and thus the verse serves as a reminder that we can't count on
Yom haKippurim to gain atonement for all sins. If you have hurt a
friend, you must first ask his forgiveness. It is not enough to turn
to Hashem.

One last reading suggests that the last word of the verse stands
alone. The verse tells us that on Yom haKippurim we will be atoned
for our sins. But then the mikra turns to each individual in Israel
and states "titharu," read not as a promise but as a command, not
"and you shall be purified" but "purify yourselves!" According to
this, the mikra is telling us that even though Yom haKippurim atones,
we can't sit back and watch. We have to be active participants; we
have to purify ourselves through teshuva!