Well, I feel like I'm going to repeat myself again even though I haven't posted here in a while, but maybe it'll have a different spin so it won't be completely redundant..
You can emphasize the first part of this question two ways: How do you KNOW or how do YOU know.. (not unlike "How YOU doin?") But seriously, that's kind of what it comes down to.
First of all, I find it interesting that even though the scripture at 1 John 5:13 emphasizes knowing over belief, as I believe one other poster on the thread has at one point, the word belief is what's thrown around for the most part, that should tell you something.
Now as for what "YOU" know, of course that usually mean 'how does one know' such and such, so it is subjective. Most people seem to confuse data with knowledge, and there is generally a lack of awareness as to the assumptions made in their argument, whatever their position on a given issue. Maybe there's a third way to emphasize this, and that is HOW. HOW do you know? I don't care who YOU are, after all HOW do I know when it's just an alias of someone on the internet? But if you break it down as far as what your reasoning is, I can see if it's reasonable. But it does seem that people are very hung up on the YOU, which of course really means ME. So other people don't believe what you believe, isn't that to be expected? Why do we become so emotionally invested with this stuff, is it really about going to heaven or is it about YOU?
In the past when I've discussed topics like religion online I would often run into people who ask "So what are you??" (to which I'd like to reply "I'm Batman" but that's neither here nor there..) But of course they want to identify me in some way, like if I'm a Christian, Buddhist or whatever. Well, I've found that often the most exemplary individuals of these groups are ones that don't identify themselves as such, and it's because they prefer to identify themselves by their behaviors or not at all, because it's not about them as an individual. Secondly, often these individuals will be silent as to questions like this, because after all the answer would have to come from YOU wouldn't it?? So why are we all trying to answer it for other people??