Evangelical Protestant Christians have been one of the primary groups opposing Mormons since the beginnings of Mormonism in the 1820s. This thesis is an examination of the historical basis for Evangelical opposition to Mormonism and the impact of that opposition on Mormon identity. This study is divided into three chronological chapters representing the nineteenth, twentieth, and twenty-first centuries in America. Evangelical animosity towards Mormonism was grounded in the Christian heretical tradition begun in the second century AD. Because of this tradition, Evangelicals were inherently afraid of heresy for two main reasons: temporal treason and eternal damnation. Due to the heterodox claims of a new prophet and new scripture, Mormonism was quickly labeled as dangerous, not only to Christianity, but to America as a whole. This perceived danger only grew as Mormonism continued to differentiate itself further with the practices of polygamy, communalism, and theocracy. In the nineteenth century, Mormon assimilation of Evangelicalism primarily affected the social structures of marriage, economics, and politics. In the twentieth century, Mormon assimilation of Evangelical identity would focus more on the incorporation of Evangelical ideology and theology. As Fundamentalism and Neo-Evangelicalism protested Mormonism as a cult, Mormonism became more Fundamentalist and Evangelical by nature, especially as the Church of Jesus Christ of Jesus Christ of Latter-day Saints recognized how such opposition negatively impacted American public perceptions. Such changes included the development of Mormon neo-orthodoxy with its emphasis on the sovereignty of God, the depravity of man, and salvation by grace. In the twenty-first century, a group of Mormon and Evangelical scholars engaged in the practice of interfaith dialogue developed by Liberal Protestants and Catholics. As part of their dialogue, Evangelicals retained the purposes of evangelism and apologetics thereby qualifying the dialogue as a new more subtle form of Evangelical opposition to Mormonism in the twenty first century. As Evangelicals continuously opposed Mormonism as a Christian heresy, such opposition effected changes within Mormonism, changes that have led to some degree of assimilation and even adoption of several elements of Evangelicalism. The most recent part of this assimilation process has been the development of Mormon progressive orthodoxy that emphasizes anti-sectarianism, anti-liberalism, and revised supernaturalism.