I'm not sure if this adds any value at this point, but...
I don't think Christians are in any different situation than the rest of us. We're all subject to the "human condition." How do any of us "know" when we're talking to someone, especially when we can't see them.
I think a while back Phatboy compared talking to God like talking to your mom on the phone. How can you know that's REALLY your mom on the other side. Well... like Faith mentioned, the things your mom says has to match with what you'd expect her to say, and how you'd expect her to say it, based on past experiences. She has to know things that mom should know. She has to speak kindly sometimes, and scold you for not calling sooner. Her voice has to be mom's voice. If it's all those things, it's mom.
Is it possible any of us are hallucinating? Sure. Is it possible it's not mom on the other line? Sure.
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In Japan, there's been a famous run of "it's me" phone calls. People call elderly people and say "It's me. I've had a terrible accident, and I need you to send me cash immediately. Otherwise I'm going to be put in jail etc. etc. etc." Due to cultural practices, I guess this isn't uncommon, because lots of people do it--they immediately send money to whatever bank account is specified, thinking that the caller is a son, or granson, or whatever. We're all subject to the same problems that Christians are.
Is it scary? Yeah, it's scary because Christians have a book where they learn that in the past, God has asked for these bad things. But I think we each have situations where we'd do bad things. If my girlfriend called and told me she needed my help, that somebody was attacking her, I'd run right over. If I happened to meet somebody when I got there, I'd attack. Cops shoot people with fake guns, people who aren't trying to run but are perceived as being aggressive. There's all sorts of this kind of stuff.
I guess I don't see why you're more concerned about this in Christians than in yourself. What's the difference? And is this for just Christians, or for all religions that have God's that communicate with people?
Ben