The Franciscan, John Peckham, was appointed to archiepiscopacy of Canterbury at the beginning of 1279. As a result of the fact that a register of his letters survives in Lambeth Palace Library, we know quite a lot about him. This was published in three volumes by Charles Trice Martin between 1882 and 1885. Included within these are several letters relating to the Jews. One of these, issued on 2 November 1281 is particularly interesting. In it Peckham noted that a number of apostates of both sexes (quod nonnulli sexus utriusque) had reverted to Judaism. This is an important point, because whilst Church doctrine permitted conversion to Christianity, there was no mechanism for a Christian (convert or not) to convert – conversion was very much a one way street. Thereafter, in a phrase reminiscent of that in Proverbs (“As a dog that returneth to his vomit, so is the fool that repeateth his folly” 26:11), Peckham remarked that these apostates returned to their vomit (ad vomitum redierunt) and their Jewish superstitions (superstitionem Judaicam). The real thrust of the letter is that Peckham wanted an enquiry, headed by the archdeacon of Rochester, set up and to use the secular authority upon these individuals.
Registrum epistolarum fratris Johannis Peckham, archiespiscope Cantauriensis, ed. Charles Trice Martin (Cambridge: Rolls Series, 1882), i, p. 239. This is available online: http://gallica.bnf.fr/ark:/12148/bpt6k503464/f350.item.zoom