Why can't we all just get along?
The problem lies what "a good person" means. In order to be a good person according to the Mormon faith, you have to be a missionary. Now suppose that in order for [insert population living where Mormons usually go to spread their faith] to be a good Hindu for example, they must not worship outside their religion/be converted.
Or for a simple example: Evangelic Lutheran-Christians (Just the split up name says a lot about division in religion) may think that in order to be a good person, one should embrace all God's children, wether they be women or gays or even unwed teenage couples. Now compare to the Christian movement in Uganda, who thinks that accepting/tolerating homosexuality is wrong and punishable in itself, while E L-C may think that it's wrong NOT to accept/tolerate it.
Had there been one objective truth in terms of what a good person is, this wouldn't be a problem.
So in order to try and get along, The idea of what makes a "good person" needs to be establish universal ideas of what is a good person? that would be that hard, I think, I mean, we could probably get a good idea by just taking all the various commandments, etc, Including input from various non-religious groups