The operative word there is fairly. You can never know anything for sure. Even scientific fact is not fact. For all you know, everything you know is completely untrue. It could all be an illusion.

Until your theory is "verified", if it every truly can be(which it can't,) it's no more valid or invalid than any other theory.

Yes, I believe in evolution. I would say our beliefs are very, very similar. However, I'm not foolish enough to think that I'm any more right in my beliefs than anyone else is. I can choose to believe one thing over another based on "evidence", but there is no way that I can say that Christians are wrong, or Muslims are wrong, or Scientologists are wrong.

I don't think it's absurd to say you believe something. I think it is quite absurd to say you know something. There's a pretty distinct difference.