The Judaica Department of The Royal Library in Copenhagen, Denmark, contains a copy of a letter from the year 53, written by Rabbi Jochanan ben Zakkai, who warns the Jews in Rome against Paul and Christianity. It was thought that the original letter belonged to the learned Leopold Immanuel Jacob van Dort from The Netherlands, who took the letter with him to the holy community of Gogin - גוגין – presumably Cochin on the Malabar Coast in South Western India. However, judging from the content and later ideas and particular words, the letter must be much younger and cannot have been written by Jochanan ben Zakkai. The manuscript itself contains an autograph by the scholar Salomo Dubno, presumably from around 1800. The analysis of the letter shows that it is composed according to the classical rhetorical pattern and that the main purpose is to warn the Jews against apostasy and especially to encourage them to keep their Jewish faith. It has not proven possible to date or place the letter with certainty, but it might be as late as from the 18th century. Hopefully, another scholar will some day be able to solve the enigmas of this remarkable letter, which falls within the tradition of Medieval Jewish polemics against Christianity.