In the Christian middle ages, people who refused to accept what they were supposed to believe about God or His Church were labelled ‘heretics’. But who decided whether their views were ‘orthodox’ or ‘heretical'? ‘Heretics’ typically saw themselves as the true Christians. Most did not dispute the central doctrines of Christianity itself, but challenged the Church as it was constituted in their day. In this they clashed with the vested interests of clerics, and as a result they typically had doctrinal and moral crimes attributed to them in the sources. These possibly bore little relationship to what they actually believed. Given this, how can we know that they were heretics in the first place? Through examining the testimony of witnesses giving evidence to inquisitors in southern France in the 1240s, this article demonstrates not only that the role of the Roman Church was contested, but also that core orthodox doctrines, such as belief in one God, in His creation of the world, and in His Son Jesus suffering and dying in human form, were disputed. Such concepts were not simply imposed on the written record by inquisitors. We have evidence of sincerely held dissenting beliefs, which were often the result of discussion and debate between three competing religious systems prevalent in the region: Catholicism, what historians now call ‘Catharism’, and ‘Waldensianism’. Inquisitors did not simply impose an externally constructed or stereotypical ‘heretic’ onto its records. The evidence indicates that ‘Catharism’ was both real and antithetical to other belief systems.