What was the idea behind manifest destiny?
that all africans are inferior to the americans and thus have no other societal role than to be slaves
that america was the most powerful world nation and had a duty to mediate all world conflicts
that a christian america had the responsibility to teach the rest of the world its religion
that it was right for americans to extend their way of life from the atlantic to the pacific

The idea behind Manifest Destiny was that it was right for Americans to extend their way of life from the Atlantic to the Pacific.

Explanation:

The doctrine of Manifest Destiny is a phrase and idea that expresses the belief that the United States of America is a nation destined to expand from the coasts of the Atlantic to the Pacific. This idea is also used by supporters to justify other territorial acquisitions. The supporters of this ideology believe that the expansion is not only good, but also obvious (manifest) and accurate.  

honestly it just comes down to the

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.