Why was there so much conflict between and among so many religious groups

The difference of opinion and in particular with opposing opinions is a source of much conflict between and among so many religious groups. This derives from the sense of "entitlement" which comes from, in particular, monotheistic beliefs. As one group gathers in belief in one way and one god and that one god's system of rules, it naturally follows that that group will determine by logic, that all opposing beliefs are wrong, and that therefore the other religious groups are wrong in their beliefs. From this can arise conflict when one tries to overpower the other to have a bigger voice or control.

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