(I'm aware that 'crazy' is pretty loaded language for people with mental disabilities. I'm using it in the colloquial sense; if that's a problem for anybody, let me know and I'll probably change it.)
It's a reasonable question: how do you talk to people you think are crazy? I'm not mainly talking about people who disagree about matters of established fact, but people who have a basic preference that you simply can't understand. Say you're a committed fan of chocolate milk (or Jesus) and you meet somebody who's an equally committed fan of unflavoured soy milk (or Joseph Smith). If you can't find the slightest shred of common ground, what on earth do you do?
Step 1: Back Away Slowly
If you can't find common ground with this person, then don't! Go and talk to somebody else about chocolate milk and the Incarnation. You'll have a better time, and so will they. It's very unlikely that you'll be able to convince this person of anything, and you need to realize this as soon as possible. As David Hume said, you can argue about how things are, not how things ought to be. The instant you start using the magical cancer-curing properties of cocoa beans to argue that your preference for chocolate milk is objectively superior, you're trying to justify the unjustifiable. Would you like it if they used the excellent prose of Orson Scott Card  to convince you that Mormons were right? Probably not.
Step 2: Ask Questions
So you've tried to get away from this person, but you can't. Maybe they're your significant other's parents; maybe they're your son. Maybe they're Superman, and they're following you with super-speed! In any case, you're forced to have a conversation - a conversation you should start by asking questions. You know that you're not going to convince them of anything, but you can't assume that this soy-milk-drinking Mormon feels the same way. If you make flat statements about your belief in the love of Christ, it's likely to come off to them as an attack. Far better to ask them about the health benefits of soy protein and sit back.
Step 3: Keep It Friendly
At some point in the conversation, chances are that either you or them are going to get angry about something and want to argue. This is a terrible idea! Unless one of you is engaged in some kind of logical contradiction - you believe in the absolute value of limited government, say, but you want abortion to be illegal - there's no possibility of making headway. It's tempting to assume that the other person is engaged in a contradiction, but don't: if there exists any consistent set of principles (a reflective equilibrium) that could justify their behavior, assume that they hold those principles. This is a practical point - think how silly you'll look if they turn out to be consistent after all.
These steps are common sense, and I wouldn't have mentioned them at all if these kind of pointless debates didn't happen every day (on the internet and elsewhere). On the internet, backing away slowly always works, so next time you're tempted to get in a huge fight, go outside instead. Or read a book. Or do literally anything else.
 This is, of course, a matter of opinion.