Who decides/rules if a law is on conflict with the Constitution?

The Supreme Court has the final say in any law making/Conflict

the number of calories in each pack of cookies has a greater variation than the number of calories in each pack of crackers.

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.