Divine Comedy of Dante Alighieri (1265-1321) is chiefly known to us for its worldwide literary fame, and is generally considered to be characterized by its medieval view of the western world. However, when we pay attention to theological background of the Divine Comedy, especially of its Purgatory, we can recognize that the theological thought of Dante not only represents the medieval western world view, but also holds true in the Byzantine tradition, which has been maintained in the Greek Catholic Church in the Central Europe. We can explain this mystery if we take notice of the history of two Councils: in the Second Council of Lyon (1274), the same theological issues were discussed as in the later Council of Ferrara-Florence (1438-39). The issue of the "Purgatory" or the "Purifying Fire of the Purgatory" was already discussed in the Second Council of Lyon, and the delegates from the Byzantine Empire, which was ruled then by the emperor Michael Ⅷ Palaeologus (1261-82), reached an agreement on this issue. Although the agreement in the later Council of Ferrara-Florence was invalidated by the Orthodox Church soon after that Council, a minor party of the Eastern Church, chiefly found in the Central Europe (the Greek Catholic Church), was true to this agreement. So since we can find the same theological view both in Dante and in the Greek Catholic Church, we can surmise that Dante was acquainted with the issues discussed in the Second Council of Lyon. One of the chief leaders of the Greek delegates in the Council of Ferrara-Florence was (later) Cardinal Bessarion (1403-72). He donated main important manuscripts of the Greek Classics to the Library of Saint Mark in Venice, and these manuscripts have greatly benefited classical scholars. Thus, both the Greek Catholic Church and the Classical Studies have their origins from Bessarion. We can say that all of the endeavors of Bessarion for the union between the Western and the Eastern Churches were made for the purpose of revalidation of the agreement made in the Second Council of Lyon. So in these points of view, too, Dante will be counted as the "sixth" (Inf. 4,102) in the line of the divine poets starting from Homer, whose best manuscript (Venetus 454: "A" of the Iliad) is found in the Library of Saint Mark in Venice.