It would be a bit of an issue for me I think. Depends on what parts of their religion they take to heart though really, I doubt I'd want to be with anyone who thought it was proper and correct for me to go to hell and be miserable for all eternity because I don't agree with them, for example. I don't see why someone being a theist of some form would on its own be a deal breaker, but a lot of major religions come with philosophical positions I can't see co-existing with my own, or that I can't see co-existing with any sort of genuine care for me
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We clasped our hands, our hands in praise of a conquerors right to tyranny