Evangelicals and other orthodox Christians wrestle with this question over at Scot McKnight’s Jesus Creed blog. McKnight writes:
To define “evangelical” we need to pay attention to those who have made it their life study to come to terms with this movement, and two scholars have done just that: Mark Noll in the USA and David Bebbington (The Dominance of Evangelicalism: The Age of Spurgeon And Moody (History of Evangelicalism) ) in the UK. They agree on this: an evangelical is a Christian Protestant for whom the central ideas are the leading authority of Scripture, the necessity of personal conversion, the centrality of the death of Christ on the cross as a substitutionary atonement, and the importance of a life of active following Jesus, seen in such things as Bible reading, prayer, church attendance, and deeds of compassion and justice. That is the standard definition of evangelical. This definition summarizes those who care about getting this term accurate. It is not a definition designed to exclude some of whom they are worried. It’s big tent definition, but it bears no ill-will toward others.
There was a time when I thought a great deal about how to define “evangelical,” but so many other scholars have done such a good job of defining the term that I have decided to just rely on them. I agree with McKnight: Bebbington and Noll are the best. For an accessible history of eighteenth-century transatlantic evangelicalism I strongly recommend Noll’s The Rise of Evangelicalism: The Age of Edwards, Whitefield, and the Wesleys.