Arianism has been called the "archetypal Christian heresy": it denies the divinity of Christ. In his masterly examination, now augmented with a new introduction and a fresh conclusion, Rowan Williams argues that Arius was in fact a dedicated theological conservative whose concern was to defend the free and personal character of the Christian God.

According to Williams, Arius's "heresy" grew out of his attempt to unite traditional biblical language with radical philosophical ideas and techniques and involved, from the start, issues of authority in the church. Thus, the crisis of the early fourth century was not only about the doctrine of God but also about the relations between emperors, bishops, and ascetical "charismatic" teachers in the church's decision-making. Williams raises the vital wider questions of how heresy is defined and how certain kinds of traditionalism transform themselves into heresy.