The 17th century was marked by intense theological debates between Protestants and Catholics, set against a background of political tensions, wars, and rivalries in Europe, in which confessional affiliations often served as markers of political division. In theology, one of the central issues concerned the doctrine of the Real Presence in the Eucharist, which was rejected by several Protestant denominations. In this context, the publication by A. Arnauld and P. Nicole of "La Perpétuité de la foy de l'Église catholique touchant l'Eucharistie" (1669-1674, t. 1-3), written as a polemic against Calvinist theology, assumed a broader dimension. The work incorporated a collection of testimonies from members of the Eastern clergy, who were presented as allies of the Catholics in this doctrinal dispute. In order to obtain such testimonies, the authors relied on the assistance of representatives of the French king in the Ottoman Empire, most notably the ambassador Ch.-Fr. Olier, marquis de Nointel. French diplomats requested patriarchs, as well as clerics, theologians, and intellectuals whom they encountered in the East or with whom they corresponded, to produce written confessions of faith attesting to their views on the Eucharist. Some of the assembled original documents are preserved today in the National Library of France, where they remain part of the former Bibliothèque du Roi. A lesser-known group of texts, probably formerly kept in the library of the Abbey of Saint-Germain-des-Prés, was acquired during the French Revolution by the Russian diplomat P. Dubrovsky and subsequently transferred to Saint Petersburg. These manuscript materials make it possible to compare the surviving originals with the texts published by Arnauld and Nicole. They are essential for understanding how the authors worked with the original documents, incorporating them into their polemical work.