The first book-length treatment of its topic, this study is aimed at abolishing the old cliche that Congregationalism failed to adapt to the democratizing culture of the westward migration. Drawing on hundreds of previously unused letters, journals, and sermons, the author argues that Congregational missionaries were aggressive evangelists who successfully adjusted to the egalitarian demands of the early republican frontier. Keepers of the Covenant critically examines the various explanations for the decline of Congregationalism after the American Revolution, and in the process, overturns generalizations that have prevailed for years. The conclusion offers a reinterpretation of Congregationalist decline that challenges much conventional wisdom about church growth. It will interest not only church historians and students of early republican America, but also sociologists and all those concerned with the decline of the Protestant "mainline" today.