Kol Nidrei

The declaration made before the recital of Kol Nidrei [We permit prayer with those
who are transgressors] has its roots in the events that occurred at the time of the
forced conversions imposed upon Spanish Jews. The Church subjected Spanish
Jewry to harsh and cruel persecution, forcing them to renounce Judaism and accept
Christianity. There were many among the Jews who, unable to withstand the cruel
treatment, publicly accepted the new faith even though they continued to practice
Judaism in secret, each one of them in his own hiding place, afraid to reveal their
faith to others. All year these anusim [forced converts] refrained from gathering
for religious worship, but on the night of Yom Kippur, they risked their lives and
gathered in secret basements to accept upon themselves the sanctity of the day and
to plead for Divine mercy for having appeared to be transgressors all year, for it
is said that God never abhors the prayers of a multitude even if those praying are
transgressors. It was in reference to them – those who were forced by circumstance to
become transgressors – that the declaration was inserted into the prayers preceding
Kol Nidrei.
This declaration was passed down to us, for in our times too, there are many who
come to the synagogue, who transgress throughout the year…
The concept of blending the prayers of willful transgressors among Israel together
with the prayers of the rest of our nation can be compared to the blending of
chelbenah with the other prescribed ingredients in the preparation of the ketoret
(incense) in the Beit ha-Mikdash (Temple). Our Sages (Keritot 6b) taught: Any fast
that does not include the wicked among Israel [as part of those who fast] is not a
fast, for though the odor of chelbenah was foul, it was listed in the Torah as one of
the required ingredients of the ketoret. And Rashi says: “Learn from this that it
should not appear to us unworthy to include in our midst – in our fasts and our
prayer – the transgressors of Israel, so that they be counted among us” (Rashi,
Shemot/Exodus 30).

Rabbi Eliyahu Kitov, The Book of Our Heritage, Vol. I, pp. 83-84 – “Permitting praying with
transgressors” is based on the Spanish Inquisition.