Two of the book’s editors provided standard definitions, In an introduction to a 1984 book, Marsden said “we may properly speak of evangelicalism as a single phenomenon” with “conceptual unity” around five points: “the final authority” of the Bible, the “real, historical” character of God’s work recorded in the Bible, “eternal salvation only through personal trust in Christ,” “the importance of evangelism and missions,” and “the importance of a spiritually transformed life.”
That last point, often mis-characterized, does not require a dramatic moment of “born again” conversion or commitment. People in biblically conservative churches are often “transformed” gradually, but thoroughly.
Bebbington’s 1989 history of British evangelicalism defined the “special marks” as “conversionism, the belief that lives need to be changed; activism, the expression of the gospel in effort; biblicism, a particular regard for the Bible, and what may be called crucicentrism, a stress on the sacrifice of Christ on the cross.”
Confusingly, both the Marsden and Bebbington criteria depict not some distinctive evangelical ideology but ardor for pretty much what all of Protestantism stood for till recent times.
The Religion Guy advises writers to refine those definitions by adding traditionalism in doctrine and morals. Thus evangelicalism embraces the ancient belief in God as the Trinity (excluding “Oneness” Pentecostals, Latter-day Saints and Jehovah’s Witnesses despite some evangelical-like traits), and on morality opposes such innovations as openly gay clergy and same-sex marriages in church.
Note that movement-wide definitions omit certain sectors’ enthusiasm for end-times scenarios or attacks on evolution. Also they involve religious substance, not politics…