The fruit of my learning in a class on the history of evangelicalism, which I wrote on Facebook during a discussion on "Deconstructing Evangelicalism":
Evangelical is a decent adjective but a crap noun.
I can show you a Presbyterian, a Pentecostal, a Mennonite, a Roman Catholic, a Quaker, several types of Baptist, but I cannot show you an Evangelical. I can show you someone who thinks they are an Evangelical, but then I can also show you someone who thinks they are an Evangelical who thinks that that other person is not an Evangelical. And I can show you someone who thinks etc. In other words, the noun "Evangelical" does very little work. It is a word without a concrete referent. The reason evangelicals think they can make up Christianity (to borrow Hauerwas's phrase) is that evangelicals are themselves made up.
Evangelicalism is real, but real as a sort of phantom tradition whose origins nobody can point to, whose contents nobody can agree on, and whose future is discussed as if the future of Christianity depends on it and its "leaders".
I haven't read Hart's book, but far be it from me to let that get in the way of agreeing with his conclusion: the emperor has no clothes.