How did the medieval Church cope with the existence, both in its past and its present, of dissent and heresy within its own body? The churchmen who were engaged in writing anti-heretical treatises in the twelfth and thirteenth centuries did not view the Church’s doctrinal history as a process of interplay between new, and possibly heterodox, ideas which defined and refined those ‘orthodox’ doctrines which became acceptable to the Church. Still less did they conceive of it in terms of Bauer’s ‘competing orthodoxies’, one of which eventually became dominant. For these polemicists, the Pauline injunction – Oportet et haereses esse in its Vulgate form (I Corinthians 11.9) – was interpreted as meaning that there must always be heresies among them. Heresy had existed as a separate entity from the inception of the Church; indeed, it was viewed almost as God-given, part of God’s scheme and the natural life of the Church, one of the four temptations sent to test and mould her. Moreover, although the heresies which had troubled the Church at various times sometimes seemed to be only distinctly related, polemicists held firmly to the conviction that all of these apparently distinct heresies were in fact offshoots of the one heresy. Their understanding of the Church’s doctrinal history, therefore, was of the intermittent manifestation, in a variety of guises, of this ‘heresy’ and its subsequent detection and repulsion by the Church. In looking back on this long history, polemicists were able to use past heresies to identify contemporary sects as heretical. At this level retrospection offered a means of combating the appeal of the ascetic and evangelical groups which were springing up during this period, many of which displayed an alarming potential to evade the control of the institutional Church. The retrospective example of the great heresies – the Arians and the Manicheans, for instance – thus provided a simple but effective method of warning the laity away from groups which the clerical and episcopal hierarchies found suspicious or threatening.