I had sudden insight that the Great Schism occurs after the rise of Islam. I wanted to investigate how much EO have been influenced by dhimmitude and Islam in general (seeing as how the EO view Protestantism as a step away from themselves).
I started on Wikipedia, and (as such things usually work out) ended up reading on Iconclasm.
Some might try to view iconoclasm (or the iconomachy - "icon conflict") through a Reformation lens. However, I don't think it can be seen that way.
The records are somewhat scarce (the iconolaters or iconodules won the struggle, and purged the iconclasts). The first phase is connected to 726, when the Byzantine emperor (Leo III) is said to have ordered the removal of icons (religious images).
It's unclear how dedicated to this cause Leo was, as later historians seem to have targeted him as the fall guy for later events. It does seem Leo's son (Constantine V) was more dedicated to the cause.
Constantine went so far as to summon an ecumenical council, although he stacked the deck with iconoclastic bishops.
What's most remarkable is that it was Constantine's son's widow who had iconclasm overturned. There was another council, this time enshrining icons. The wording used is most remarkable:
"As the sacred and life-giving cross is everywhere set up as a symbol, so also should the images of Jesus Christ, the Virgin Mary, the holy angels, as well as those of the saints and other pious and holy men be embodied in the manufacture of sacred vessels, tapestries, vestments, etc., and exhibited on the walls of churches, in the homes, and in all conspicuous places, by the roadside and everywhere, to be revered by all who might see them. For the more they are contemplated, the more they move to fervent memory of their prototypes. Therefore, it is proper to accord to them a fervent and reverent adoration, not, however, the veritable worship which, according to our faith, belongs to the Divine Being alone — for the honor accorded to the image passes over to its prototype, and whoever adores the image adores in it the reality of what is there represented."We see here an attempt to make a distinction between "reverent adoration" and "worship".
There was a second period of iconoclasm (814-842), which was again overturned under the guidance of the king's widow.
Perhaps most interesting, this all does not seem to have arisen from any particular theological crisis or revival.
The Muslims had started to invade the Byzantine empire, and they had an extreme aversion to religious imagery. It appears some in Byzantium attached their military troubles to God's displeasure. It was thought that eliminating these idols would restore God's pleasure (and allow them to defeat the Muslims).