In the Great Synagogue of Vilna before World War II, on the second day of the holiday of Shavuot, the congregation would recite a prayer in honor of Count Walentyn Potocki, a nobleman who, according to local legend, had converted to Judaism and was burned at the stake in punishment on that day in 1749. The count’s gravesite—destroyed with the rest of the cemetery by the Soviets—was frequently visited by the pious on the fast of the Ninth of Av. But the lack of corroborating contemporary documents has led scholars to cast doubts on the tale, and the website of Vilnius’s official Jewish community, following Wikipedia, dubs it a “myth.” Yosef Vilner argues that skeptics shouldn’t be so quick to dismiss it:
As we read the [Wikipedia] article, we are informed that: . . . “the Polish historian Janusz Tazbir asserted that the story originated at the turn of the 19th century and was published in a Jewish periodical issued in London as The Jewish Expositor and Friend of Israel (vol. 8, 1822).” [But] The Jewish Expositor and Friend of Israel was a monthly periodical published by the London Society for Promoting Christianity amongst the Jews, by no means a “Jewish periodical.”
The abovementioned volume contains “Extracts from the Journal of Mr. Wolff,” who was a Jewish convert to Christianity. . . . In the spring of 1822, he met with Rabbi Menachem Mendel of Shklov, one of the leading rabbis in Jerusalem at the time. A neophyte Christian and a fervent missionary, Joseph Wolff initiated theological discussions with Rabbi Menachem Mendl in a disguised attempt to convert him to Christianity. Rabbi Rabbi Menachem Mendl, on the other hand, intended to bring Joseph Wolff back to the faith of his forefathers.
Rabbi Menachem Mendel, who was one of the outstanding disciples of the Vina Gaon, [i.e., the famed talmudist Elijah Kramer], immigrated to Eretz Israel in 1808 and settled in Jerusalem in 1816. . . . There is little doubt that he heard the story [of Potocki] from rabbis of Vilna who were contemporaries to the trial and the execution in 1749.
As to Tazbir’s claim that executions on religious grounds were rare, Vilner notes that
in the span of a five-year period from 1748 to 1753, another two such executions occurred in Poland. Abram Michelevich, a Jew from Mohilev, and his Christian partner, Paraska Danilowna, were executed in Mohilev in 1748, Abraham for proselytizing and Paraska for apostasy. And on June 2, 1753, Rafal Sentimani was burned alive for having converted from Catholicism to Judaism on the outskirts of Vilna.