Deliberate mistake: In the scene with the forest battle, the scene with flaming arrows being fired was filmed on grass (look at the ground) in a separate location, because it wasn't safe to fire them in a real forest. (01:33:25)
Deliberate mistake: Marian is wearing pantaloons, or knickers, in the rape scene. Knickers were not worn by women until Victorian times, and then they were split at the crotch.
Answer: The white robes, reference to "the old ways", and pentagram across the map when the Sheriff meets the Barons suggests per-Christian Druidism; the upside down crucifix certainly implies Devil-worship. These two spiritual paths are, by nature, mutually exclusive. In short, a fictional pseudo-witchcraft invented for the film, yes.