"you should believe He is the One True God, since He said He is"
Well, that's just political posturing and group psychology.
When a group is recruiting members, they are interested in having high barriers to membership, and in denigrating other groups and creeds.
It's just not politically expedient for a relgious group to say their God and selected beliefs are not only God/religion/worldview. That's politically weak because it allows for e enical credence in other Gods/religions/worldviews, absolutely the last thing a political religious group wants to do.
Of course, these "We're Number One" religious types have been programmed to believe that their religion is 100% pure, true, and exclusive of all other religions, and has no political history or objectives. And the biggest political hustle and recruiting tool is that outside of Christ, you go to for eternity.