The mainline Protestant churches are a group of Protestant denominations in the United States and in some cases in Canada largely of the theologically liberal or theologically progressive persuasion that contrast in history and practice with the largely theologically conservative Evangelical, Fundamentalist, Charismatic, Confessional, Confessing Movement, historically Black church, and Global South Protestant denominations and congregations. Some make a distinction between "mainline" and "oldline", with the former referring only to denominational ties and the latter referring to church lineage, prestige and influence. However, this distinction has largely been lost to history and the terms are now nearly synonymous.

Christian fundamentalism, also known as fundamental Christianity or fundamentalist Christianity, is a religious movement emphasizing biblical literalism. In its modern form, it began in the late 19th and early 20th centuries among British and American Protestants as a reaction to theological liberalism and cultural modernism. Fundamentalists argued that 19th-century modernist theologians had misunderstood or rejected certain doctrines, especially biblical inerrancy, which they considered the fundamentals of the Christian faith.