Question: "What are the mainline denominations?"
Answer: When people speak of “mainline denominations,” they are usually referring to the historic, established Protestant groups. In 2009, George Barna published a list of the churches that are commonly considered “mainline”: American Baptist Churches in the USA, the Episcopal Church, the Evangelical Lutheran Church in America, the Presbyterian Church (USA), the United Church of Christ, and the United Methodist Church. Other sources add the United Church of Christ, the Christian Church (Disciples of Christ), Friends (Quakers), and some other churches to the list.
The mainline denominations are so-called because they are thought to represent the oldest, most influential branches of Protestantism in the States. Other denominations have derived from the mainline denominations. For example, the Presbyterian Church of America split from the Presbyterian Church (USA) in 1973.
Outside of the mainline denominations are the evangelical denominations or fellowships. There are several differences between mainliners and evangelicals. The mainline denominations are, as a rule, more theologically and politically liberal; they hold a more “modernist” theology, viewing the Bible as a historical document that may not be inerrant and is not “inspired” in the sense that it is the actual words of God; they lean toward neo-orthodoxy; and they are prone to ecumenism, the social gospel, and the acceptance of homosexual clergy.
Evangelicals, on the other hand, uphold the inerrancy, verbal-plenary inspiration, and authority of Scripture; they emphasize the importance of a born-again experience through faith in Christ; they encourage evangelism; and they are prone to autonomy in church government, Christian education, and Fundamentalism. In the last six decades, the mainline denominations have seen a marked decrease in membership, while most evangelical churches have held steady or increased membership rolls.
Author Timothy George comments on the “liberal Protestantism” of the mainline denominations with a warning against straying from biblical authority. At the root of the compromises within the mainline denominations are “a broken doctrine of biblical authority, a loss of confidence in the primary documents of the Christian faith. The patina of pietism and the lushness of a well-rehearsed liturgy are no substitute for what the Thirty-nine Articles calls ‘the sufficiency of the Holy Scriptures.’ Apart from such commitment, it will not be long before other cardinal tenets of the Christian faith become negotiable, including the Trinity, the full deity and true humanity of Jesus Christ, and redemption wrought through the sacrifice of Jesus on the cross. . . . The church and the Bible are coinherent realities in the economy of grace. One will not long survive intact without the other” (“3 Lessons from Crisis and Decline in the Mainline”).