How to discuss religion with believers?

[Dr H]

So my question is: how do you engage in a discussion with these people without it derailing?

I'd say a lot depends on your motivation for engaging in the discussion in the first place.  Is the point to understand why the person believes what they do?  Or is it to mock their belief and try to convert them to non-belief through verbal brow-beating?

I've put in a lot of time studying comparative religion, and the history of religion.  My motive for this is that, like it or not, I live in a world and a society surrounded by religion and religious views.  As a practical matter, I need to understand how this works in order to survive and function in this context.  The primary other option would be to become a hermit, and my nature is a bit too social for that.

Having done this, I have been able to engage in discussion of religion with religious believers of all sorts.  I've had these discussions with everyone from strangers on the street to priests, rabbis, imams, and swamis.  Some have been friendly cultural exchanges.  Some have been heated, but civil philosophical debates.  And some have come close to fist fights and exorcisms (usually from my misjudgment of the context).  

In general, my motivation for having religious discussions at all, is to try to better understand the motivation of people around me, so that I may more peacefully co-exist.  Sometimes my motive is to hopefully persuade someone from making what appears to me a bad choice, based on their religious belief.  

Now, I do like to argue, and will admit that I sometimes have religious discussions just to scratch that itch, but I usually try to have those with someone I know well enough to know that they are not going to be offended or insulted by that kind of interaction.

Almost never has my motive been to try to persuade someone to give up their religious belief and become an atheist.  For one thing, I think this is virtually impossible:  that kind of change has to happen from within.  For another, I feel that one of the most ridiculous things I could become would be the atheist equivalent of an evangelical preacher or door-to-door salvation peddler.

That's been my experience; YMMV.  But it might give you some things to think about.

Oh, and welcome.