I was taught that after the Council of Nicea no christian church kept the Passover on the 14th of Nisan. Is there any truth to that? Is there a church in the middle ages that kept the Christian Passover on the 14th of Nisan? I've tried researching online but no luck. Thanks!
As far as I know no Christian Churches except Jewish ones kept the Passover, as it is a Jewish holiday. You probably refer to the timing of Easter.
The Council of Nicaea ruled that Easter should be celebrated on the first Sunday on/after the first full moon on/after the vernal equinox. Sometimes it matched the Jewish Passover sunday and sometimes it did not, because the Christian system mixed both solar and lunar calendars to determine the date. For more info, see