The study focuses on criminal cases instructed by the Spanish Inquisition in Sicily against crypto-Muslim slaves - "moriscos" and Christian renegades who would not deny Islam - between the 16th and the 17th centuries. It aims at the analysis of the circumstances and the different courses leading to their conversion through the study of documents of the Spanish Inquisition itself, which since its foundation in 1478 was in charge of the fight and eradication of heresy from the territories subject to the Spanish crown. Apart from Christian renegades and their experience of the court, this work is chiefly concerned with "moriscos", their relationship with the Inquisition court and especially with those who resisted its pressure and refused to reconvert to Christianity; among these, the so called "martyrs of Islam" condemned to the stake. Through its courts and a tight network of clerical and lay elements, the Holy Office kept under control and punished any deviance of Christian believers from the teachings of the Church of Rome and any attempt of Muslims to secretly practice their faith. Inquisition sources from Sicily and Spain have been fundamental to delineate the lives of slaves and renegades and observe the presence of "moriscos" on the island, the various stages of their emigration from Spain, their servile or free conditions, their tendency to hide their faith in order to avoid persecution.