When a historian comes across a papal letter or any other document relating medieval heretics, one of the first questions that inevitably arises is ‘Where does this information come from?’ Of course, the pope did not know the intricacies of every heresy, nor had he met every heretic. The information he was able to garner was shared in a continuous stream of data between the higher echelons of the Church and the ecclesiastical men in the field: this led to the creation of a careful and distinct image of the heretics. The textual representations provided in papal letters were collected and reported both in contemporary and in later texts regarding heretics and inquisition – that is, in treatises and handbooks. In order to understand the construction of knowledge and information about the heretics we should focus on the narratio: a distinct narrative part of a papal letter which set out the backgrounds and the reasons that led the pope to write that specific document. Starting from the few elements gathered from the people fighting the heretics at the forefront, the pope shaped a specific depiction of the heretics and their beliefs – which then became the Church's official image.

It is well known that the image of medieval heretics went through a long process of development, which reached its peak in the first half of the thirteenth century. This process was linked to the concurrent and progressive criminalisation of heretics and, therefore, the development of more repressive judicial instruments to eradicate them. A turning point was the papacy of Gregory IX (1227–41), the period in which many historians set the birth of the medieval inquisition. During this nascent phase, neither the information about heretics nor the procedures for dealing with those found guilty of heretical behaviour were clearly defined. Papal letters can therefore be seen as evidence providing a unique insight into the shaping of the inquisitio haereticae pravitatis. Through these letters, Gregory IX could establish and develop the judicial practice of the men who were entrusted with anti-heretical tasks. Because no legal procedure had been formally established each assignment and its modus operandi were individualised, dictated by each case's unique political and social context.