I set the bar of knowing at things like our own existence, the existence of objects like the computer I'm typing on, things that have as yet an uncontested rate of verifiability. The only thing we can know with 100% certainty is each our own existence, but there's still a big gap between knowing something like that this is God's creation, and knowing that my car is real. The point is that it's more of a very strong belief than knowledge.
I don't know about that. For you to say "I know my car is real" implies that you know a great deal about metaphysics. For example, you know that objects endure through time, don't wink out of existence when you're not looking at them, that you're not just a "brain in a vat" as it were, and so forth. You also assume the reliability of your sensory and cognitive apparatus when you make experience this vaunted "bar" of yours. As Hume and other skeptics have pointed out, these are rather breathtaking leaps in the dark. Your so-called "simple belief" that your car exists is in fact a rather shaky claim. And when we consider the matter philosophically, it's hard to see how that claim is much stronger than my claim that God created the universe. The bottom line is that your knowledge claims embed a huge number of unprovable assumptions about how the world (if there is one) works, how your cognitive faculties work, and even about the very function of your cognitive faculties. It's therefore inaccurate to say that your so-called direct beliefs are more reliable than mine.
For I say I know God by experience. In order to say that, I have to appeal to certain metaphysics, including the function and operation of my cognitive faculties, along with the same assumptions you make. In short, the distance between my belief in God and your belief in your car isn't that great. And if my belief in God underwrites my belief that God created the universe, then the status of my belief in God's creation is assured.
As for verifiability, I have no way to verify that you are typing on a computer. You might be posting using a cell phone or a blackberry. So it's unverifiable to me. Does that mean I don't know you aren't using a computer? If not, verifiability isn't worth much as an epistemological standard.
Then, why do they call it the Holy Spirit? How do we know it's not they who have experienced the "real" Holy Spirit, and you who have experienced an "impostor"?
Different religions use the same terms, but the terms denote different things. I use scripture, my religious community, and my ancient religious tradition to help me discern between the Holy Spirit described in the bible and other spirits. It's too much to do on my own. But that doesn't answer your question how YOU might know that I've experienced the real Holy Spirit. Well, to know for certain you must enter into a life of discipleship. It starts with repentance from sin and confession of Jesus as Lord. At that moment, the Spirit of God enters you and you start to get to know him. The scriptures, together with a living community of believers and a 3500 year-old tradition will assist you. Apart from that process, there's no knowing.
Two things: One, you say that the Christian theology is internally-consistent. I can point out some inconsistencies, but I don't think we should go that direction here, as we might already be too far off-topic. If you want to start another thread, we could discuss that. Two, you say that you are more convinced by Christianity than other religions. Again, this doesn't sound like you "know" to me. It sounds like you really believe in it, but the language here is much less committal than your statement that you know.
To your first point: I'm sure you can point out putative inconsistencies. For our purposes, let's confine my comments about internal consistency to the theology concerned with Christian epistemology. To your second point: Would it have been better if I had said "much, much more convinced"? You still labor under the misguided apprehension that admitting the real possibility of error entails a lack of knowledge. That's simply a non-sequitor. I know God exists and has created the universe. I also happen to know that there are other theories out there, and my commitment to my theory is fraught with risk. It could be mistaken. But I honestly don't think so. Christianity does so much better than its rivals at explaining things and fitting me for life in this world than any other rival that I've encountered and studied with any degree of earnestness (and that's a fair few). And so I put my faith, my full confidence, in it rather than any of the alternatives. I'm making a knowledge claim alright, but I'm also trying to remain humble in the process. If knowledge and humility are truly at odds, then I'll take humility over knowledge. Luckily, I don't have to choose between them.