The article establishes that the role of the Rus’ Archbishop Peter at the Council of Lyon (1245) is not limited only to the transfer of sensational information about the Tatars, as is usually thought. No less important in their consequences was the demonstration of readiness to resume negotiations on the union of Churches, which continued with varying success throughout the 13th century between the Apostolic See and the rulers of the Byzantine (Nicene) Empire. In the mid-1240s, the Rus’ Church hierarchs who headed the Kiev Metropolis of the Greek Church played an important role yet underestimated by researchers in establishing direct contacts between the Greek and Roman Churches and the resumption of negotiations on Church unity.