@JLeslie @Seek Agreed. A lot of evangelicals—or at least, the ones with the biggest megaphones—treat Christianity (and Heaven) like an exclusive club. There’s actually an interesting sermon on just that topic from the church I was raised in (though I understand if you don’t have time to read it; I don’t read many sermons these days either). Interestingly, we see less of this kind of cultural isolationism in churches with a strong history of theological scholarship (such as Catholicism and most of the older versions of Protestantism).
In Catholicism, for instance, there is the view that our experience of Heaven differs according to our personal merit (it’s good for everyone, of course, but different because some souls are better equipped to get more out of it). And in Methodism, there tends to be a strong focus on the universal nature of salvation (again, the subject of that sermon I linked). Not all Catholics and Methodists are well-versed in this part of their religion, but it does mean they get less pressure from above to think everyone else is automatically damned.
@NerdyKeith A lot of the discussion here has focused on the Abrahamic religions, but @JLeslie (and her father-in-law) reminded me of a point I meant to bring up before: not all religions are as different as we think, and not all of them are exclusive. If we look beyond our current time and place to various forms of polytheism, for instance, we can see that many people understood their dedication to a particular god or goddess as a way of securing advantages that suited them personally (whereas another person might be better off with the advantages conferred by a different god or goddess).
As far as we can tell, this is true of the Greek and Roman mystery religions. It also seems to be true of certain Vedic traditions. And a similar point of view can be found in the dialogue between Buddhism and Jainism (with sages on each side contending that both can lead to enlightenment, but that their religion does so more effectively or efficiently). This isn’t to say that there have never been “exclusivist” Buddhists and Jains (or Dionysians and Orphics, for that matter, though there aren’t a lot of them around anymore), but it does show us another way in which a difference of religion could be a non-issue in a relationship.