The figure of William L. Sullivan, the Paulist missionary and teacher who renounced Roman Catholicism in 1910 and migrated eventually to Unitarianism, poses a continuing challenge for historians of American religion. How is one to interpret his spiritual pilgrimage? Is Sullivan best understood as a reformer whose abhorrence of the perduring Vaticanism and Romanism in Catholic ecclesiology impelled him towards liberal Protestantism? Was he primarily, as he put it in his unfinished autobiography, “a moral personality under orders,” ultimately restless with every institutional expression of the “religion of the Infinite Spirit”? Or does the key to his life and thought lie in an excessive patriotism and nationalism reflected, in part, by his allegiance to the “cause” of the Americanists? Was he a Modernist? Most provocative, perhaps, is the thesis linking these two heresies: does Sullivan's career stand as the embodiment of the continuity between Americanism and Modernism?