Four incidents are of particular importance when we think about the causes of the Great Schism: (1) the quarrel between Photius and Pope Nicolas I; (2) the incident of the Diptychs in 1009 (the name of the pope was dropped from the list of hierarchs that are commemorated in the Liturgy); (3) the attempt at reconciliation in 1053-4 and its disastrous sequel; and (4) the   Crusades, one of which resulted in the sacking of Constantinople. It was actions such as these that sealed the separation of the Churches of the east and west. Soon not only the Papal claims but the Filioque became involved in the dispute. Byzantium and the west (chiefly the Germans) were both launching great   missionary ventures among the Slavs. The two lines of missionary advance, from the east and from the west, soon converged and when Greek and German missionaries found themselves at work in the same land, it was difficult to avoid a conflict since the two missions were run on widely different principles. The clash naturally brought to the fore the question of the Filioque, used by the Germans in the Creed, but not used by the Greeks. The chief point of trouble was Bulgaria, a country which Rome and Constantinople alike were anxious to add to their sphere of jurisdiction. The Khan Boris was at first inclined to ask the German missionaries for baptism. Threatened, however, with a Byzantine invasion, he changed his policy and around 865 accepted baptism from Greek clergy. But Boris wanted the Church in Bulgaria to be independent and when Constantinople refused to grant autonomy, he turned to the west in hope of better terms. Given a free hand in Bulgaria, the Latin missionaries promptly launched a violent attack on the Greeks, singling out the points where Byzantine practice differed from their own: married clergy, rules of fasting, and above all the Filioque. At Rome itself the Filioque was still not in use, but Nicolas gave full support to the Germans when they insisted upon its    insertion in Bulgaria. The Papacy, which in 808 had mediated between the Franks and the Greeks, was now neutral no longer.

While Photius was naturally alarmed by the extension of German influence in the Balkans, on the very borders of the Byzantine Empire, he was much more alarmed by the question of the Filioque, now brought forcibly to his attention. In 867 he took action.

Religion and politics have, throughout history, been closed intertwined.