The paper focuses on the politics of confessionalization and vernacular language in the context of the Ottoman Empire and Safavid Iran around the mid-sixteenth century. It presents a catechism written by a certain Gharibi in defense of Shiism. Bearing the title Hikayat-i Yuhanna, ‘The Story of Yuhanna,’ and surviving in a unique copy preserved at the Majlis Library in Tehran, the treatise is the Turkish version of a widely circulating anonymous work with various renditions in both Arabic and Persian; it features a Jew who, after he interviews four religious scholars representing the four Sunni legal schools, becomes convinced of the superiority of Twelver Shiism and converts to it. As to the author, Gharibi, we only know that he was originally an Ottoman subject affiliated with the Sufi brotherhood of Ibrahim Gulsheni, and that he may have later emigrated to Safavid Iran, writing literary works mostly in Turkish which he dedicated to Shah Ismail (r. 1501-1524) and Shah Tahmasp (r. 1524-1576). After attempting to establish the relationship between Gharibi’s work and the extant Arabic and Persian versions, the paper will analyze the piece against the background of the increasing number of popular theological treatises appearing in the era mainly in Persian and less prominently, in Turkish, as part of the crystallization of Twelver Shii orthodoxy in Safavid Iran, a process parallel to the consolidation of Sunni orthodoxy in the Ottoman Empire. Written in Turkish by an Ottoman enthusiast of the Safavids, the work sheds a unique light on the dynamic relationship between confessionalization and vernacularization in the early modern Iranian and Ottoman context.