Ablank is a person, especially the member of a group, who takes law into his own hands

Vigilante.

Explanation:

Vigilante is a form of the word vigilant, which means "keeping a watchful or close eye on events and people." Sometimes a vigilante will make news for catching a criminal, and sometimes vigilante groups form to target crimes in a bad neighborhood. A common combined use is "vigilante justice," which refers to a form of law-keeping outside of the official channels of police and the courts.

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I believe it is vigilante

Although I am not quite sure because I do not know the background context to this question.

the catholic church withhold people's access to the bible during the middle ages.

people could go to church and listen to the teachings, but it was absolutely forbidden to own and/or read the books of both the old and new testaments. it was also prohibited to translate the books (which where originally in biblical hebrew) to any other language.

this was done so the church could remain in control of what their teachings were and to enhance their importance and power, while also being able to judge people's actions through information that was unavailable for the general public.