On their way to Northumberland, the Priestleys stopped in Philadelphia, where Joseph gave a series of sermons that helped promote the spread of Unitarianism. According to J. D. Bowers, who studied Priestley's influence on Unitarianism in America, "a decade Priestley served as the inspiration and leading force in the spread of Unitarianism in America and the formation of numerous societies that followed his teachings on congregational formation, the education of youth, lay preaching, and espousing one's faith in the presence of opposition from (and to) both the Protestant majority and a competing liberal faction." Through Priestley's influence, at least twelve congregations were founded in Maine, Massachusetts, New York, Vermont, Pennsylvania, Virginia, and Kentucky, including the First Unitarian Church of Philadelphia and Northumberland's Unitarian Universalist Congregation of the Susquehanna Valley. When he preached, Unitarians and non-Unitarians flocked to hear him and his sermons were published throughout the country. During his years in America, Priestley became increasingly convinced that the Millennium was approaching. His close study of the Bible, together with the happenings in France, persuaded him that he would see Christ's return.