Complex sentence: The Old Believer Priest Group were known as Theodesians who believed in Jesus of Nazareth, King of the Jews (JNKJ) and who had prospered in Russia and they had found favour with the Tsars and the Imperial Government vis-à-vis the Non-Priest Group.
Split into simpler sentences: The Old Believer Priest Group, known as Theodesians, believed in Jesus of Nazareth, King of the Jews (JNKJ) and had prospered in Russia. They found favour with the Tsars and the Imperial Government vis-à-vis the Non-Priest Group.