From David Cramer, a very insightful approach:
When it comes to defining “evangelicalism,” much has been made of the so-called Bebbington Quadrilateral. Named after evangelical historian David Bebbington who identified the quadrilateral, the Bebbington Quadrilateral defines evangelicalism by four characteristics: biblicism, crucicentrism, conversionism, and activism. This quadrilateral has been used as a baseline definition of evangelicalism by many since Bebbington introduced it in the 1980s. But it is not without its problems. Some have suggested that it is too generic: many Christians could identify with these four points without thereby identifying as “evangelical.” Others have suggested that it is too narrow: some might identify as evangelical without fully endorsing one or more of these four points. In other words, the definition seems a bit deductive rather than being derived from the self understanding of those who identify as evangelical. (This is not the place to enter such debates, so don’t shoot the messenger!)
In his article on evangelicalism, Erdel offers a slightly different (though perhaps compatible) definition. According to Erdel, evangelicalism is identified by two theological marker and three behavioral markers (the Erdelian Pentagon?).
Of the theological markers, Erdel writes, “The core theological definitions of evangelicalism were settled during the Reformation and have endured ever since. . . . Evangelicals are born-again Bible-believers. ‘Jesus loves me, this I know, for the Bible tells me so.’ Thus Evangelicals strongly affirm the two great insights reflected in the material (sola fides/sola gratia/solus Christus) and formal (sola scriptura) principles of the Reformation.”According to Erdel, then, evangelicals are those who:
(1) agree with Luther that salvation is by grace through faith in Jesus
(2) believe this because they believe it is what the Bible authoritatively teaches.
If this were all there is to evangelicalism, then it would basically be synonymous with Protestantism (as, indeed, the term is sometimes used outside of North America). However, Erdel combines these two theological traits with three behavioral ones for his more fulsome definition:
For Erdel, evangelicals are also those who:
(3) “have traditionally been more concerned with personal piety and moral purity than many ” other Christians;
(4) “have traditionally been very active in their support for evangelism and foreign missionary outreach”;
(5) “have had a strong bent toward low church sensibilities, decrying formal, sacramental worship, championing instead spontaneity.”
In sum, the Erdelian Pentagon defines evangelicals as low-church, pietistic, biblicist Protestants who actively support evangelism and missions.
Based on Erdel’s definition, large swaths of Anabaptist-Mennonites are indeed evangelicals, whether they know it or not.