About Mother Teresa--if she doesn't have to believe in God why should anyone else have to?

And--how can you be unsure of the very existence of the supreme being, yet AT THE SAME TIME claim to know his exact position on contraception, what kind of food to eat, what kind of sexual acts humans ought to perform, etc... Wouldn't that kind of detail be even more profoundly unknowable? Yet one can be a fundamentalist in the worst way, as MT was undoubtedly--and agnostic too?