Basicallt крестьянин and христианин are similar etymoligically.Originally Posted by ingberlin
I imagine it had something to do with identifying and distinguishing Ruthenian (Russian/Ukrainian/Belarussian, whatever) Slavs from the other peoples who they lived alongside, e.g. the Jews, Muslims (Tatars), etc. In the past the idea of nationality wasn't developed, so simply people (i.e. peasants) identified most closely to their language and religion as a form of self-determination rather than the modern notion of belonging to a state.
So the word isn't that strange afterall.