The article presents and analyzes information from letters of missionaries of the Franciscan Order Guillaume de Rubruk, Giovanni Montecorvino and Monk Johanka, as well as notes and treatises of a missionary from the Dominican Order Ricoldo da Montecroce, about Nestorians in the Genghisid states. The author comes to the conclusion about the value of the works of these authors as sources on the history of Nestorianism, which contain data on the doctrine and religious practice of the followers of East Syrian Christianity and their relations with Catholic missionaries.
