I think contemporary Christian Evangelicalism is misconstrued by most of the rest of us as being an aberrant form of historic Christianity. I think that is a mistake. It's hardly Christianity at all, as the world has known it, for good or ill, down through the centuries. To be sure, it arose out of Christianity, but psychologically, it seems be a New Age feel-good cult. These people are hippies gone bad. (And I say this as an old hippie, gone bad in a different direction.)
It's true that the roots of the modern evangelical movement lie in the remnants of 19th century revivalism, the 2nd Great Awakening. The ecstatic conversion experience accompanying the big emotional-breakdown camp meetings became a marker, a sort of test, for authentic salvation (Augustine, Aquinas, Luther, and Calvin would, of course, all twirl in their graves.) That mid-20th century evangelical religion eventually made it officially so, of course, was in effect to ditch nineteen centuries of Christian theology and belief.
But for all its celebration of ecstatic conversion, 19th century Protestant revivalism had not yet forgotten the great questions of historic Christianity which defined sectarian boundaries within the Christian mainstream; questions of the nature of God and the nature of Christ; the mystery (or more strictly speaking, incoherence) of the Trinity; the intricate Rube Goldberg linkages of grace, works, faith, free-will and predestination; of the nature of the sacraments and of the structure and authority of the church and its hierarchy; and the history, nature and authority of the scriptural canon.
All that is pretty much gone. By and large modern evangelicals know absolutely nothing of any of it, regardless of where the nominal roots of their own lineage lie. Many evangelical sects, but not all, are descended from Calvinist churches, but contemporary evangelical eyes glaze over when predestination is mentioned. Some of the Pentecostal and Holiness groups are descended--historically, but not in any other meaningful way--from Wesleyan anti-Calvinist churches. Most of the current members of these groups have never heard of the controversy between Wesley and the Calvinists, and many of them have never heard of John Wesley.
Sad to say, there are even Catholic evangelicals, who share with their nominally Protestant brethren a total ignorance of Church history and doctrine.
Contemporary Southern Baptists (like other evangelicals) seem to know all about the Rapture, a bizarre doctrine first appearing full-blown in the mind of a 19th century (non-Baptist) preacher named John Darby, but know nothing of the traditional Baptist resolution of questions of theology, soteriology, and hierarchy in the brilliant--and in a way admirable--twin Baptist cop-outs of soul-competency and the priesthood of the believer, which, for all their unorthodoxy, at least have some antecedents in nineteen centuries of Christian mysticism and several centuries of anti-hierarchical radical Protestantism.
So what have the evangelicals replaced historic Christianity with? Well, like cowbird eggs in a magpie's nest, we find the Rapture and allied weird nonsense like dispensationalism and Christian Zionism, indeed a whole bubbling cauldron of crackpot millenarian woolgathering based on obsessive readings of the Apocalypse of St. John; not to mention fundamentalist scriptural idolatry as a sort of enabling background radiation. Lately we also find Republican Kulturkampf politics.
But above all that, the ecstatic conversion experience is firmly established as the feel-good centerpiece of their feel-good religion, to which, to be fair, all the Darbyist gibberish is actually secondary.
And the fact that the warm glow of conversion is now psychologically central to Evangelicalism and has as a practical matter replaced almost the entire baggage of former Christian orthodoxies has drawn vast numbers of the Boomer generation into the evangelical fold, like bees to nectar. And there are lotsa bees out there. (Sadly, they tend to vote, and vote Republican, more due to the cooptationist success of shrewd Republican strategists than to an inherent Republican-ness within Evangelicalism itself.)
Personally I think the cosmic convergence of hysteric frontier immigrant camp-meeting emotional release with New Age if-it-feels-good-it-must-be-true Boomer complacencies has come to fruition. The Age of Aquarius seems to be here.