The Mainline are the Seven Sisters of Protestantism: The Episcopal, Congregational/United Church of Christ, United Methodist, Evangelical Lutheran, Presbyterian, American Baptist, and Disciple of Christ denominations. . . . During the colonial period, the Mainline consisted of the Episcopal/Anglican Church and the Congregational/Puritan Church. In the early nineteenth century, these denominations were joined by the evangelical churches of the Baptist, Methodist, and Presbyterian persuasion, who grew in size and prominence because of the democratic impulse of the First and Second Great Awakenings. Joined by the end of the century by the Disciples and Evangelical Lutherans, we have the Seven Sisters.
Lantzer asserts that the term mainline “should mean we are talking about the majority,” but is also a matter of “cultural engagement and influence.” On the grounds that the term has evolved in the past, he proposes redefining the mainline so that it includes most Christians in America today: adding evangelicals, non-denominational Christians, Roman Catholics, and Pentecostals.
As an aspiring ecclesiologist, I don’t think much of this idea! I think the description mainline functions primarily as a theological, not a sociological, signifier. There are meaningful theological commonalities among the mainline Protestant churches that are not shared by Roman Catholics, Pentecostals, or non-denominational Christians. (I’m not sure about “evangelicals,” as that term seems to have several meanings; and I’m not sure where the emergent/emerging church movement would fit.)
Interestingly, the commonality is not one of polity: I see episcopal (governance by a bishop), presbyterian (governance by a vestry, essentially a committee), congregational (governance by the entire congregation, essentially a committee of the whole), and connexional (it’s a Methodist thing) in the Seven Sisters.