John was an undergraduate and postgraduate at Cambridge where he obtained his PhD in church history. He was also an ordained Methodist minister although he made his career as a trenchant critic of both religious and academic orthodoxies.

When the Department of Theology was founded at the University of Bristol in 1965, he was one of the first appointments made by Kenneth Grayston, the founding professor.

John became one of the leading modern church historians of his generation, a fact recognised by the university which promoted him to a personal chair in Theology in 1977. He was thus the Department's second ever professor.

John retired in 1988 and was appointed Emeritus Professor of Theology.

He wrote a pioneering study of Victorian Christian revivalist movements entitled Holding the Fort: Studies in Victoran Revivalism (1978) and the best short biography of William Temple, Archbishop of Canterbury.

His last major work was Wesley and the Wesleyans: Religion in 18th Century Britain (CUP 2002). Despite his pugnacious and iconoclastic publishing style, he was a gentle, modest and kind man and a great supporter of younger academics and students.