Two images of heresy and orthodoxy prevailed in Christian antiquity. One had been canonized by Eusebius and was generally accepted until the seventeenth century, that orthodoxy is primary and heresies are deviations, corruptions of a previously pure, virgin orthodoxy. Its echo is distinctly audible in the edict issued by the court of Ravenna in 418 condemning the teaching of Pelagius, Celestius and their followers: they were confounding the ‘light of catholic simplicity shining forth with permanent radiance’. This image represented orthodoxy as a given constant: a rock buffeted by the waves, the light of the sun hidden by the clouds. Held often alongside it, there is another, equally common, belief with an even more venerable ancestry (1 Corinthians 11:19): that heresy serves to bring orthodoxy to light. Compared with the first model, this treats heresy as creative: orthodoxy is the product of faithful response to heresy. Both models invite us to see the conflict of heresy and orthodoxy in the perspective imposed on it by the ‘orthodox’. The besetting temptation for any historian is to take the past at the valuation of those who emerged as the victors; and nowhere more so than in the study of what councils, theologians and ecclesiastical historians have come to label as ‘heresy’ and ‘orthodoxy’. What is ‘heresy’, what is ‘orthodoxy’, and what constitutes ‘progress’, are all determined by the victors, that is to say by those who were, by their own definition, the ‘orthodox’.