Joel Watts has written a couple of posts inquiring into what defines “Evangelicals” and “Mainliners” and how does one distinguish between the two: see What are Mainliners (Compared to Evangelicals and Fundamentalist) and ARE there dividing lines between Mainline and Evangelical? I think these are great questions and I recommend leaving your comment(s) on one or both of these posts. I’ve noticed quite a few blog posts circulating around the Internet suggesting that the recent controversy where World Vision went from affirming benefits for and acknowledgment of their employees who are in same sex marriages to renouncing that stance the next day has caused some to either seriously doubt their affiliation with the label “Evangelical” or denounce it altogether. Those who declare themselves to be leaving Evangelicalism for Mainline Christianity assume that there is a black-and-white divide between the two, but this isn’t necessarily true. Many Mainliners would call themselves Evangelical while many Evangelicals may affirm doctrines (or reject others) that would allow them to easily align with the Mainline.
Of course, this returns us to the problem of definitions: What is an Evangelical? I’ve asked myself that question several times on this blog. I don’t think I reached a conclusion. Do all Evangelicals find themselves outside of so-called “high-Church” traditions? Nope. There are Evangelical Anglicans, Methodists, Presbyterians, etc. Do all Evangelicals affirm sola scriptura? Not sure: there seems to be a resurgence among Evangelicals in affirming Creedal Christianity because the uninterpreted canon doesn’t necessarily result in obvious Trinitarianism or the dual nature of Christ or the personhood of the Holy Spirit or many other essentially Creedal dogmas that Evangelicals want to retain even as they declare sola scriptura. What about inerrancy? No. There are whole Evangelical institutions of higher learning that don’t require one to affirm inerrancy as an identity marker as well as many Evangelical parachurch organizations, non-profits, and local congregations, of course.
Maybe the quest for a singular definition is misguided? Maybe “Evangelical” means a dozen things, sociologically, although it seems to mean only one blurry thing theologically: people of the Gospel. Mainline is a tad easier since only various traditions often with European roots and contemporary expressions influenced by the evolution of the United States can be included.
Rudolf Bultmann argued in his attempt to defend the Jewish people in Germany prior to WW2 that, “Understood properly, a people is not a biological, but rather a historical phenomenon, and therefore our participation in it is a matter not of descent, but of existence.”  Whether or not this is wholly I accurate I do not intend to debate, but there may be something applicable to this discussion. Maybe a people are not defined wholly by doctrine as much as “a historical phenomenon”. What makes a Methodist in the UMC different from a Wesleyan in the Church of the Nazarene? What differs a Presbyterian in the PCA from the PC (USA) from those groups splintering off of the PC (USA) now, including independent congregations? Sure, doctrine is part of it, but mere history may be more central. If this is so, Evangelicals are all those who think the starting point for doctrinal formation and ecumenical dialogue must be an attempt to understand the meaning of the Gospel in today’s world. If this is true, than an Evangelical may be part of the Moral Majority or the Emerging Church. An Evangelical may be someone like N.T. Wright, who formerly served as an Anglican Bishop or John Howard Yoder of the Mennonite tradition. Evangelicalism’s “history” may be beginning over and over again as different groups and new generations decide that it is OK for them to walk down the street from their Baptist Church to visit their Pentecostal neighbor in order to ask, “What is it that we both proclaim?”
 Theologische Enzyklopädie, 65, cited in Konrad Hammann (2013-01-16). Rudolf Bultmann: a Biography (Kindle Locations 6648-6650). Polebridge Press. Kindle Edition.