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.
Chosen answer: The masked men weren't part of a cult, they were the Sheriff's soldiers, the same ones that Robin meets when he first returns home. They are just in hoods and masks to appear intimidating while trying to kill the noblemen, like Locksley.
Bishop73