In 1054 “some papal legates to Constantinople became embroiled in a bitter argument with the patriarch Michael Cerularius.

In 1054 “some papal legates to Constantinople became embroiled in a bitter argument with the patriarch Michael Cerularius. Cerularius insisted on traditional Byzantine practices in churches throughout the empire, especially in Armenia, where unleavened bread had long been used in the Eucharist as it was in the West. Exasperated by the patriarch’s intransigence, the papal legates excommunicated him. These personal condemnations did not end all communion between eastern and western churches, but they ruined the emperor’s alliance with the papacy and raised intractable issues that plague any relations with the West.” 

The issue is not that there really was a great schism, it is fairly clear there was not. Relations seem to have recovered quite a bit after this, after all these were just papal legates getting angry over long-standing differences. Really the problem was that this dispute offered a religious pretext for later events, animosities, and violent actions towards the Romans by the Catholics. It was not until the crusades solidified the divide as beyond repair that this schism looks “great” in hindsight.

Source: A History of the Byzantine State and Society by Warren Treadgold

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