In 1288 the Dominican Riccoldo da Monte Croce headed to the East, where Pope Nicolas IV and the Master General of his Order had sent him on a mission to the Nestorians and Jacobites. For this mission he stayed about ten years with Nestorians, Jacobites and other Eastern Chris-tians. The account of his sojourn in the East, entitled Liber peregrinationis, and the theoretical anal-ysis he derived from it – his Libellus ad nationes orientales – allow us to understand how a thirteenth-century Roman Catholic missionary perceived the relationships between the Catholic Church and Oriental, non-Chalcedonian Churches, in the time of the Second Council of Lyon and shortly after the definitive loss of the Crusader states. Riccoldo’s rhetoric is influenced by several ele-ments – the pontifical definition of Christianity, Dominican missionary theories and the experi-ence of his stay among the Nestorians and the Jacobites in Niniveh and Baghdad. Riccoldo da Monte Croce learned Arabic and Chaldean during his long residence as a Roman Catholic mis-sionary in the East, where he gained a deep knowledge of Eastern Christians. Never did he lose or downplay, however, his identity as a mendicant friar and an apostolic missionary. In the life and the works of Riccoldo da Monte Croce, we witness the confrontation of a convinced Do-minican missionary with Eastern Christians, which leads us to rethink the definition of Christiani-ty grounded in the notions of unitas and uniformitas.