The reforms of the patriarch Nikon concerned mainly rites which were out of synch with those used elsewhere in the Orthodoxy (f.e. Russian now had to cross themselves with three fingers instead of two) as well as the iconographic and orthographic conventions used in Russia as the time (f.e. instead of "Isus", Jesus had now to be called "I'isus" to conform to the spelling in Greek - ιησους). He made his first order right after having become a patriarch in 1652 but since that lead to protests in the clergy he called the other patriarchs for a Synod. In 1656 Patriarchs of Antiochia and Nicea, as well as the metropolites of Moldova and Serbia arrived and gave Nikon their support in that the then-current Russian rites could not be regarded as orthodox.

The sum of changes to be made were then distributed in a printed book through all the churches and monasteries which caused protests in the common folk, the nobility and the clergy.

The archpriest Avvakum made quite the patriotic case that if the Russian tradition was declared heterodox all previous Russian saints and statesmen could not be regarded as orthodox either. Moreover he noted that Russia of all Orthodox lands could least of all be told of to have fallen out of grace with God. He was exiled at first but since he kept writing letters denying the possibility of salvation to all who fell to what he termed the "Nikonian Heresy" he was burned at stake in 1682.

The patriarch Nikon himself had stepped down back in 1658 and was subsequently stripped of his titles in the church.

Sources:

N.A.Kostomarov - Russian History in Biographies of its main figures - Patriarch Nikon

The Living of Archpriest Avvakum written by himself

Muscovy

Muscovy was Russia since Ivan the Terrible "married the Tsardom" in 1547.