This paper sketches a broad outline of the relationships governing fundamentalism, modernism, and Pentecostalism in the first half of the twentieth century in the United States. Fundamentalism and Pentecostalism were tangentially aligned through a common nineteenth century evangelical ethos via the holiness movement. Modernism shared with Pentecostalism certain aspects of Pietism but diverged dramatically in its rationalistic approach to Scripture. Fundamentalism and modernism agreed in this rationalist endeavour but parted over the role of the supernatural in Christianity. A mutual distrust of modernism and their shared evangelical ethos led ultimately to cooperation between fundamentalists and Pentecostals by the beginnings of World War II.