In the summer of 1630, Nicolas-Claude Fabri de Peiresc (1580-1637), a magistrate, cleric, and tireless correspondent in the South of France, offered to negotiate the release of Thomas D'Arcos (1573-1637?) from his Moorish captors in Tunis. Peiresc had a pragmatic reason for writing. As an intermediary in the Republic of Letters and collector of curiosities, he needed information from North Africa that D'Arcos could provide. But to Peiresc's dismay, D'Arcos converted following his release from captivity, perhaps the only Frenchman to do so. Many converts published captivity accounts after their return to their country of origin. D'Arcos's letters provide a unique insight into his dual existence both in Tunis, where he gained local prestige as a convert, and in France because of his ability to procure information from North Africa. An examination of 80 published letters exchanged between Peiresc (Aix-en-Provence and Belgentier), D'Arcos (Tunis), and a mutual friend Honoré Aycard (Toulon) in the period 1630-1637 reveals the way in which these correspondents framed the conversion at a time when such an action was considered an “apostasy.” D'Arcos presented a paradox by living in two worlds. He never justified his conversion but instead insisted that his inner convictions (faith) remained unchanged even though his dress, or “habit,” had changed. Peiresc avoided confronting the issue of the conversion and addressed D'Arcos as if nothing had changed, using strategies to lure him back to the Catholic faith. He dissimulated news of the conversion in the Republic of Letters but at the same time shared observations obtained by a source he identified as a “former captive.” The exchanges with the intermediary Aycard were more explicit, and correspondents disclosed their feelings concerning the impact of the conversion on their relations as well as on the broader community. Although D'Arcos expressed a fear that he had lost Peiresc's respect, he did little to comply with the Frenchman's need for specific information, blaming any shortcomings on Barbary and providing only the exotic rather than the noteworthy.