Kind of an interesting question, depending on perspective. Because I’m sure there could be at least a dozen answers I could come up with–and that does not begin to count the answers other people could give at the exact same time. – and most of them are not the historical phenomenon I am actually talking about.
The historian and life-long student in me wants to pontificate on double faith; that being the particular manifestation of Orthodox Christianity that sprung up after the church swept through eastern Slavic lands and became the dominant religion of the ruling class. The rulers pushed their new holy religion on the masses, regardless of what the masses themselves believed or wanted. Rulers prerogative, as it were, of course.
Which led to a duality of belief that came down to us as a particular phenomenon in early modern Russian religious belief:
Dvoeverie – Double Faith
Just what is this?