I am working on a paper that I have tentatively titled "Feeling Mainline." This comes from a dataset that grouped people into big categories based on their denomination, including dividing Protestant denominations into Evangelical, Black, and Mainline. This is a fairly standard move in sociological studies.
Later in the survey, though, they also asked each respondent how well different words described their religious identity, such as "religious liberal" or "fundamentalist." Among the words they asked about were "evangelical" and "mainline."
The most interesting finding: about a quarter of members of mainline denominations think of themselves as "evangelical", and about a quarter of members of evangelical denominations think of themselves as "mainline." In fact, about a third of black Protestants see themselves as mainline, and even 15% of Catholics say "mainline" describes them very well.
Clearly, people are not using the term the same way sociologists do.
I think embracing the identity "mainline" means that you feel entitled to participate in decisions affecting your community. This is more than just "mainstream" (as opposed to weird, deviant, extreme, eccentric, etc.) I think "mainline" carries a sense of responsibility for the world.
I use this odd phrase - entitled to participate in decisions affecting - because this is how Annette Lareau describes the sense of entitlement that middle and upper-middle-class children learn from being raised according to what she calls "concerted cultivation." Where poor and working-class kids are taught to be obedient and do their work - what Lareau calls the "natural growth" method of childrearing - middle class parents work to develop all of the talents of each of their children through all the methods of pushing, coaching, and enrichment they can muster. And one aim of concerted cultivation is to teach children to be independent and curious, to actively promote their own growth.
Middle class kids, thus, grow up with a sense of entitlement. But this entitlement is not to get whatever they want, but to participate in decisions that affect their lives.
Which, I believe, is how people who feel mainline in their religion have a sense of entitlement to participate in decisions that affect their communities.