I recommend that you go to the library and get books on Lithuania. Lithuania rose as the political representation of pagan Europe and remained pagan and polytheistic until late in the medieval period. They successfully conquered territory held by the Catholic regimes in Poland and Sweden as well as the Orthodox regime in Russia. The regime is somewhat prefigured by Grand Prince Vladimir's attempt to fashion a state religion in Russia around the deity Perun before he abandoned this project in favour of converting to Christianity.
I'll also echo others' comments about the syncretic nature of medieval Christianity especially in places which were converted late in the medieval period like Russia.
Finally, my take on medieval polytheism in the camapign I currently run is to use the Japanese and Roman models of a God-emperor on earth. Orthodox Christian monarchs as well as the 6 Christian patriarchs of the middle ages are an easy fit with a God-emperor system; thus we have not only saints but we have either the Patriarch of Constantinople or the Bishop of Rome as a kind of living God figure.
I'll also echo others' comments about the syncretic nature of medieval Christianity especially in places which were converted late in the medieval period like Russia.
Finally, my take on medieval polytheism in the camapign I currently run is to use the Japanese and Roman models of a God-emperor on earth. Orthodox Christian monarchs as well as the 6 Christian patriarchs of the middle ages are an easy fit with a God-emperor system; thus we have not only saints but we have either the Patriarch of Constantinople or the Bishop of Rome as a kind of living God figure.