Other mistake: When the outlaws trap and rob a carriage in the forest, they put hooks on the back of it to keep it in place. But when the driver spurs the horses on faster, they run off, pulling the driver off the carriage and away behind them. There's no logical reason for this to happen, as you see the horses attached to the carriage, there is no evidence, either audio or visual, of the harnesses breaking, and in any event there really isn't enough force exerted for the harnesses to break to begin with.
Other mistake: Michael McShane's name is spelled wrong ("Micheal") in the opening credits.
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.