The goal of this article is to show, through a case study of a convert from Islam to Catholicism in the seventeenth century, how multi-faceted and complex the phenomenon of conversion is, where political, social, and religious factors are intertwined. The article recounts the conversion story of Mohammed el-Attaz, later known as Baldassarre Loyola (1631-1667). Son of the king of Fez (Morocco) of the Saʿadian dynasty, Mohammed was captured on his way to Mecca by the Knights of Malta; he converted to Christianity, went to Italy, became a Jesuit, and spent some years of his life converting Muslims in Italian port cities. The story of Baldassarre Loyola is unique for many reasons. First, this is the only known case of a Muslim prince joining the Society of Jesus. Second, we have an extraordinary range of sources: more than 200 letters written by Baldassarre, Christian and Muslim first-hand sources, an unpublished Autobiography, and a 600-page unpublished Life written by Baldassarre’s spiritual director, the Jesuit Domenico Brunacci. Additionally, a sacred drama on Baldassarre’s story (El gran principe de Fez) was composed by Calderón de la Barca and performed in Jesuit colleges in Europe as well as overseas. This case study of a man between two worlds—struggling for a new identity but always linked to his ancient roots—illuminates, through the phenomenon of conversion, the tormented, rich, and fascinating relationship between Islam and Christianity on the eve of modernity.