What is the complete subject of this sentence? The sun is at the center of our solar system?

The complete subject contains the main noun in the sentence; the predicate contains the verb, or action, in the sentence

the answer is the government.

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.