In 1666 Comenius wrote a dedicatory epistle, addressed to the Ottoman Sultan, which he intended to be printed with a translation of the Bible into Turkish: in it, he expressed an unusually conciliatory attitude towards Islam, while also defending the Christian Bible against the Muslim accusation that it was a corrupted text. This article prints the text of the epistle and discusses the background to it. Comenius's attitude towards Islam is examined, across the range of his other writings. His belief that the Turks could and would be converted to Christianity is also connected with the apocalyptic anti-Habsburg prophecies of Mikuláš Drabík, which Comenius promoted. And his defence against the charge of textual corruption is located in a tradition of argument which can be traced back, via the Calvinist Johannes Hoornbeeck, to the response made by a Roman Catholic, Filippo Guadagnoli, to an anti-Christian text written by a Persian scholar in 1622.