The history of the ecclesiastical breach between East and West, and of the attempt to heal it, is very long and obscure. There was the Photian schism of the middle of the ninth century, but that was over within a few years. Two centuries later the Patriarch Cerularius was excommunicated by an over-zealous papal legate and retaliated in kind; but that quarrel was not final. The Fourth Crusade captured Christian Constantinople and never went any further, but set up a Latin kingdom there which lasted less than sixty years and did as much to antagonize the Greeks as anything else. Yet, a little more than ten years after he regained his throne, the restored Greek Emperor—but not the Greek Church—had accepted the Latin faith and union in the Second Council of Lyons (1274). His purpose, however, was political, to win the Pope’s help to prevent any attempt to re-establish the Latin kingdom of Constantinople.

From then on over the next century, whenever Constantinople was more in danger than usual by reason of the rapid advance of the Turks, the Byzantine emperors approached the pope of the time, as head of western Christendom. They asked for military aid, and they offered the possibility of union of the Churches through the medium of a general council. The offer was, of course, a sort of bribe, but not entirely so. Both East and West sincerely deplored the schism that divided them and would have wholeheartedly welcomed a genuine union.