Dr. Martyn Lloyd-Jones sought to answer that question in a series of three addresses delivered in Austria in 1971. (You can read these talks in the book, Knowing the Times.)
Kevin DeYoung has been blogging through the lectures:
- What Is an Evangelical? Part 1 (first address on the dangers to avoid in defining the term)
- What Is an Evangelical? Part 2 (second address on guiding principles and general characteristics)
- What Is an Evangelical? Part 3 (third address on essential truths vs. secondary truths)
- What Is an Evangelical? Part 4 (Kevin’s appreciation and concerns)