The apocryphal Latin text Interrogatio Iohannis was the “ catechism” of the Cathars of Concorezzo. This written work is particularly well contextualised. It is preserved in two Medieval manuscripts, attached to the end of a xiith century Latin New Testament (Viennese manuscript) and being part of a collection intended for the Inquisitors (Dôle manuscript). Several antiheretical Italian treatises provide information on the Cathar bishop Nazaire, who possessed this secret written treatise imported from Bulgaria towards the end of the xiith century. Finally, the evidence of Anna Comnena (Alexiade) and the treatise against the heretics of Euthymius Zigabenos allows one to recognise the close relationship between the myths of the Interrogatio and those of the Bogomils of Byzantium from the beginning of the XIIth century.