Mainline Protestantism is not a church. It has no shared confession, no governing council and no single form of church government: the Episcopal Church keeps bishops and dioceses, the Presbyterian Church (U.S.A.) keeps elders and presbyteries, and the United Church of Christ keeps its member congregations fully autonomous. What binds these bodies together is a classification, not an institution. The Pew Research Center's Religious Landscape Study sorts American Protestant respondents into one of three traditions based on the specific denomination they name: evangelical, mainline, or historically Black Protestant. A Southern Baptist is classified evangelical. A member of the National Baptist Convention, USA, is classified historically Black Protestant. A member of the Evangelical Lutheran Church in America or the United Church of Christ is classified mainline, whatever that person's own personal theology happens to be. The category groups denominations, not individual beliefs.
Historian William R. Hutchison gave the tradition's most institutionally prominent members a name in his 1989 essay collection Between the Times: the Seven Sisters of American Protestantism, seven denominations that dominated American religious, civic and educational life through the mid twentieth century. Compared with the evangelical tradition, mainline churches tend to hold a less exclusive account of salvation, treat historical critical Biblical scholarship as compatible with faith rather than a threat to it, and put a comparatively public emphasis on social reform. None of that makes mainline Protestantism a single thing to believe. It names a family of institutions with a shared historical position in American public life, not a shared creed.