Arent they are basically the last european (if you stretch it with the ural-definition) ethnic group with a polytheisthic religion without a broken line of worship and oral transmission?
I was curious about their theology and wanted to get some material about it, but one of the few pamphlets that has been made by them "Onaeng Oyla-A priest speaks" got banned in russia for hurting orthodox sensibilities and inciting seperatism-is not avaible on english therefore.
"Maris and Mordvines in todays russia, language, culture identity" by Helimski, E., Kahrs is the only source in english/german I could find, but its sadly just from an observatory stance not a spiritual one.