In the 13th century, the booming missionary movement in Orient led the Dominicans to define the necessary skills for their envangelisation movement. The letters written by Prior Philip – who was in charge of the Dominican province in Holy Land – are testimony of the first efforts made by the Dominicans in the area. During the Second Council of Lyon (1274), the elements that could allow for the return of the Greeks in the Roman Catholic faith were discussed, a few years after the fall of the Latin Empire of Constantinople. The return was deemed indispensable for the success of a crusade meant to recapture Jerusalem. The former master general of the Order of Preachers, Humbert of Romans, developed in his Opusculum Tripartitum, the conditions that were necessary for the reductio Graecorum, among which were the skills required for the missionaries. Fourteen years later, Dominican Preacher Riccoldo da Monte Croce, sent among the Maronites, the Jacobites and the Nestorians in Nivine and Baghdad, enables one to measure to what extent the missionary theory was implemented.