In this article, I will examine the primary sources and historical circumstances related to Pope John XXII’s dispatch in 1333 of a new archbishop, Nicolas, to Khanbaliq (now Beijing) and attempt to solve the mystery surrounding the identity of “Rex Corum” in the pope’s letter. At the same time, I hope to provide new information on religious exchanges between the Christian West and East Asia during the Mongol era, as well as shed light on the political situation of the ruling circles in the Yuan Empire during the 1330s.