The foundations of Russian Christianity—gradual conversion; absorption of church law; native ascetic monasticism (Kiev’s Monastery of the Caves) and cults (Boris and Gleb, the Vladimir Theotokos icon); characteristic architecture; and crafting of patriotic and didactic sermons (Ilarion), hagiography (Nestor), chronicles, and pilgrimage itinerary—all hearken back to pre-Mongol Rus’. Under Mongol protection, the Rus’ Church flourished; the anti-Catholic, the late Byzantine hesychastic devotional and artistic package, representing a distinct brand of Orthodoxy, arrived, and communal monasticism, spearheaded by Sergii of Radonezh, spread. From the mid-fifteenth century, autocephalous resistance to Church Union with Rome, and unification and expansion under Moscow accompanied Nil Sorskii’s hesychastic treatise, Iosif Volotskii’s theological–didactic–inquisitorial Enlightener, and his followers’ promotion of a Moscow-centred national historiography, sacred monarchy, and pious household guide. The newly established Moscow patriarchate (1589) aided survival and resurgence after civil war and foreign intervention (1604–1619), while the practical need for better-educated Orthodox clerics from (then Polish–Lithuanian) Ukraine and Belarus contradicted pretences of native purity. Would-be Orthodox reformers in the 1640s differed over how much Westernizing education and ritual flexibility were permissible to bring Russian Orthodoxy in line with Ukraine and the Greeks. Patriarch Nikon’s high-handed liturgical reforms (1655), more ruthlessly supported by church and state power after a synod deposed him for political overreach (1666), catalysed the variegated dissenting sectarians commonly called ‘Old Believers’. The century ended with a Moscow Academy (1687–) complementing Kiev’s (1632–) and conflicting, alterative Westernizing visions among Europe-oriented elites.