The radical German Pietist Johannes Kelpius traveled to Philadelphia in 1694 with a group of early German immigrants to America. He settled on the banks of Wissahickon Creek, where he led a small separatist religious community based in esotericism and mysticism, inspired by the teachings of Johann Jacob Zimmerman.

The aura of mystery surrounding Kelpius is heightened by the scarcity of his written record, of which his Diarium remains one of the few surviving texts. The Diarium provides a detailed account of his ten-week Atlantic voyage, relating stories of the...