The author is Janet M. Hartley from LSE, here is one excerpt:
…the religious composition on the Volga is complex. Finno-Ugric settlers originally followed shamanistic beliefs, although many converted, at least nominally, to Orthodoxy after they became subjects of the Russian Empire. The ruler and the elite in Khazaria probably converted to Judaism sometime in the early ninth century. Kalmyks in the south and south-east of the Volga were Buddhists (the only Buddhists in Europe). The Bolgar state, the Golden Horse and the khanates of Kazan and Astrakhan were, or became, Muslim. the Russian and Soviet states were conscious of the potential threat of Islam in the Volga region from the time of the conquest of Kazan in 1552. The history of the Volga is, in part, the history of (often forced) conversion to Orthodoxy by the Russian government and the reaction to this of the local inhabitants. In many cases, the conversion process was incomplete or, in the case of Islam, could be reversed. The remoteness of much of the Volga countryside attracted Old Believers — that is, schismatics from the Russian Orthodox Church who did not accept the changes in liturgy and practice in the middle of the seventeenth century.