Napier regarded "A Plaine Discovery of the Whole Revelation of St. John" (1593) as his most important work. It was written in English, unlike his other publications, in order to reach the widest audience and so that, according to Napier, "the simple of this island may be instructed". "A Plaine Discovery" used mathematical analysis of the "Book of Revelation" to attempt to predict the date of the Apocalypse. Napier identified events in chronological order which he believed were parallels to events described in the "Book of Revelation" believing that "Revelation"'s structure implied that the prophecies would be fulfilled incrementally. In this work Napier dated the seventh trumpet to 1541, and predicted the end of the world would occur in either 1688 or 1700. Napier did not believe that people could know the true date of the Apocalypse, but claimed that since the Bible contained so many clues about the end, God wanted the Church to know when the end was coming.