As various Slavic populations were Christianised between the 7th and 12th centuries, Christianity was introduced as a religion of the elite, flourishing mostly in cities and amongst the nobility. Amongst the rural majority of the medieval Slavic population, old myths remained strong. Christian priests and monks in Slavic countries, particularly in Russia, for centuries fought against the phenomenon called dvoeverie (double faith). On the one hand, peasants and farmers eagerly accepted baptism, masses and the new Christian holidays. On the other hand, they still persisted performing ancient rites and worshiping old pagan cults, even when the ancient deities and myths on which those were based were completely forgotten.
This was because, from a perspective of the Slavic peasant, Christianity was not a replacement of old Slavic mythology, but rather an addition to it. Christianity may have offered a hope of salvation, and of blissful afterlife in the next world, but for survival in this world, for yearly harvest and protection of cattle, the old religious system with its fertility rites, its protective deities, and its household spirits was taken to be necessary. This was a problem the Christian church never really solved; at best, it could offer a Christian saint or martyr to replace the pagan deity of a certain cult, but the cult itself thrived, as did the mythological view of the world through which natural phenomena were explained.
While folk beliefs and traditions of all Slavic peoples indeed are the richest resource for reconstructing the ancient pagan beliefs, these may very likely have lost their original mythology and sanctity. People entertained a vague idea that some festivals must be celebrated in a certain way, some stories must be told or some songs must be sung, merely in accordance with tradition. Cults of old deities were mixed with worship of new Christian saints, and old rituals blended among new Christian holidays....