Continuity mistake: When Robin arrives on the beach at Dover he drops his 'roll' then falls onto the sand, rolling in the shallow water. The shot from behind his head shows the roll but when the camera moves to the front the roll has gone.
Continuity mistake: When Robin is talking to the Bishop in his private chambers, they're sitting by the window. They're both lit on the side of their faces that are near the window, but for some reason in the Bishop's closeups, he has harsh white light on the other side of his face as well. Not only is there no source for this light, but in the wide shot that side of his face isn't lit.
Continuity mistake: Just after Will is tied to the barrel to be executed and the drums start rolling, there's a wide shot of the whole gallows, including the barrel, but Will is missing.
Continuity mistake: When the outlaws are being attacked in the forest, one man is shot by a flaming arrow in the back. But when he falls off his platform onto the ground, the arrow is suddenly in his stomach.
Continuity mistake: When John starts to cross the burning bridge to rescue his wife and baby, he steps onto the bridge and almost into the fire, which is just inches from him. But when the bridge breaks, the fire has suddenly moved several feet below him (so it doesn't burn him up as he climbs to safety, I presume.).
Continuity mistake: As Marion begins to distract Robin from aiming the arrow you will notice that at the wide angle shot she picks up a brown fletched arrow. As the shot tightens it is a white fletched arrow.
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.