Most of them are quasi-spiritual agnostics who don't like to admit that the vast majority of major religions make claims that are mutually exclusive to those of other religions, and so they get triggered when someone tries to convince someone else that a certain religion is true or false.
Personally, I think tolerance is good, but many leftists extend it to include the forced affirmation of disagreeing view points, which is, in my opinion, both rude and stupid.