Mack was a prosperous miller in Schriesheim in the Palatinate who fell in with the radical Pietist preachers Ernst Hochmann von Hochenau and Gottfried Arnold, lost his mill and his citizenship, and moved to the county of Wittgenstein, where the count tolerated dissenters. His originality was in taking Pietist inwardness and insisting it required an outward, disciplined, baptised church — which the Pietists around him thought was a return to the very externals they had left. He wrote little: Rights and Ordinances in 1715 and Basic Questions in 1713. He arrived at Germantown in 1729 with the last large group of Schwarzenau Brethren and died there on 19 January 1735. His grave went unmarked until the nineteenth century, which is consistent with everything else about the way the church handles its own beginnings.