The mainline Protestant churches (sometimes also known as oldline Protestants) are a group of Protestant denominations in the United States and in some cases Protestant denominations 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.
Mainline Protestant churches have stressed social justice and personal salvation, and both politically and theologically, tend to be more liberal than non-mainline Protestant churches. Mainline Protestant churches share a common approach that often leads to collaboration in organizations such as the National Council of Churches, and because of their involvement with the ecumenical movement, they are sometimes given the alternative label of "ecumenical Protestantism" (especially outside the United States). While in 1970 the mainline Protestant churches claimed most Protestants and more than 30 percent of the American population as members, they are a minority among American Protestants, claiming approximately 15 percent of American adults. Some have criticized the term mainline for its alleged White Anglo-Saxon Protestant ethnocentric and elitist assumptions, and its erroneous association with the term "mainstream," since the term mainline almost exclusively described White, non-fundamentalist and non-evangelical Protestant Americans from its origin to the late twentieth century.
Mainline Protestants were a majority of Protestants in the United States until the mid-20th century.