It’s hard to decide what is most remarkable about “Byzantium: Faith and Power, 1261-1557,” at the Metropolitan Museum. It’s astonishing that the museum has once again assembled an extraordinary group of rarely or never-before-seen treasures—icons, manuscripts, textiles, architectural fragments, and more—many from obscure, remote collections in troubled places. (This is the third in the Met’s series of exhibitions examining, chronologically, the achievements of the Eastern empire of Constantinople and the Greek Christian world.) Even more astonishing than the fact that is not only that any of the diverse works in the show survived is that they were created in the first place. Even more astonishing is that any of the diverse works in the show survived or that they were made in the first place, given the turbulent history of the period under review. The opulent survey encompasses the turbulent history of the final two centuries of the Byzantine empire and the first century of its posthumous influence. The starting date, 1261, marks the restoration of Greek political and religious power to the Eastern Christian world after the more than half-century of hated Frankish rule that followed the brutal sack of Constantinople by the Venetians during the Fourth Crusade. (It wasn’t just the infidel that the crusaders went after. For evidence of Venetian looting, see, for starters, the four great bronze horses that until recently stood above the main portal of San Marco. For an immensely readable account of the complicated history of the Greek Christian empire, see John