Well, I'm not one of those people, but I hope you don't mind my putting in my two cents worth anyway since I come from a religious tradition where people are very fond of putting it that way (i.e. "I know the [Mormon] Church is true."). I don't claim to "know" anything of a spiritual nature, but I do have a very strong belief. If I were to say simply, "I believe..." it would probably not have the impact I want it to have. The best way for me to explain where I stand with respect to my religion being true is to say, "I am convinced..."
As far as a conviction that there is a God out there is concerned, I simply cannot accept the possibility that this isn't so. I've tried. I've made attempts to convince myself that our universe, our Earth, and all life on it could have come about without any kind of divine direction. I can't. I have tried and I can't do it. My mind just won't go there, any more than the minds of so many atheists I know can conceive of the existance of a Higher Power. In all honestly, I believe that our inclination towards a belief in the divine is pretty much hard-wired. That's why I don't ever attempt to convince atheists that they should believe in God.
From there (i.e. from my belief in God) forward, I have to accept what I can make sense of, what is logically possible to me. I've done this through the process of elimination. I can't explain God "logically" but I can make a whole lot of logical arguments that He is not the Trinity that was defined at Nicea in 325 A.D. Consequently, I have to reject any Christian religion that embraces that doctrine. I also believe that God is not only loving but just. A just God would not send billions of people to an eternal in Hell because they happened to have been born at the wrong time or in the wrong place to have had the opportunity to hear the gospel of Jesus Christ. So, I have to reject any religion which teaches that this is, in fact, what's going to happen, and tough ***** for the people it happens to affect. To me, a God that stresses the importance of love between a man and woman would not dissolve that relationship when we die, so I can't accept any religion that says that's what's going to happen. It makes perfect sense to me that if God spoke to His people for thousands of years through men He would call as prophets, He wouldn't just stop when Jesus Christ came to Earth, nor would He call someone a few hundred years later (i.e. Muhammed) and say, "That's it. He's the last one." He would continue to do what He did for four thousand years. I'd have to go with a religion that taught what I believe in that regard.
As to why I can't go with any of the eastern or non-revealed religions, I suppose it's that to me, if God created me, He must care about me. He would have to be a personal God in order for me to feel that He was worthy of my worship. It would pretty much have to be the Abrahamic God.
But, here's the bottom line as far as I'm concerned... If there is a God, I believe He loves us. I believe that He is merciful and just. To me, it just doesn't make sense that if it was so important to Him that we understand perfectly everything about Him and what He expects of us, He is fully capable of making sure that we couldn't possibly get it wrong. I think He wants us to get it right, but I don't think that the time He's given us to do so is going to be resticted to our mortality. I think our opportunities will continue after our bodies are laying in the ground, and I think things will be clearer to us then than they are now. And I definitely don't think that He is going to turn everyone who failed to get all of the facts right into His eternal firewood. He loves us and He created us for completely unselfish reasons.
I don't "know" any of that to be true. But I am "convinced" in my own heart that it's the only perspective that works for me.
