A committed pacifist, Muste joined the Fellowship of Reconciliation shortly after its foundation in 1916. He participated in a peace demonstration late in the summer of 1916, with US entry into the First World War looming and some parishioners withdrawing from his congregation. Pressure began to build further over Muste's pacifist views in April 1917, when the United States formally declared war on the German and Austro-Hungarian Empires. After taking two months of vacation leave in the summer of 1917, he decided that the time had come to leave. In December 1917, he formally resigned his pastorate position.