H. R. Trevor-Roper, The European Witch-Craze of the Sixteenth and Seventeenth Centuries, NY: Harper and Row (1956 / 1969) pgs 107-108

"Medieval civilization, 'feudal' civilization, was a civilization of the plains, or at least of the cultivated lands which could sustain the manor and its organization. In the poor mountain areas, pastoral and individualist, this 'feudalism' had never fully penetrated thither, or at least it had not been maintained there in comparable form. Missionaries might have carried the Gospel into the hills, but a settled Church had not institutionalized it, and in those closed societies a lightly rooted orthodoxy was easily turned to heresy or even infidelity. M. Fernand Braudel...has pointed to isolated mountain societies long untouched, or only superficially touched, by the religion of state and easily -- if as superficially -- converted to the...religion of a sudden conqueror.

The mountains, then, are the home not only of sorcery and witchcraft, but also of primitive religious forms and resistance to new orthodoxies...As [the Dominicans] carried the gospel of 'feudal' Christian Europe into the unfeudal, half-Christian societies of the mountains...[they] found that their success was transitory: that ancient habits of thought reasserted themselves..."

More material here

And even this from the online encyclopedia of the Catholic Church!