The only way to have no doubt is to be omniscient.
I think we can have certainty about particular ideas, but I think it is impossible to have no doubt about the unprovable and be intellectually honest.
Some things can be proven beyond a reasonable doubt, but does that still leave unreasonable doubt? Is unreasonable doubt relevant?
Some things can never be proven, and to me, that means they cannot be known. Not to go off on a religious tangent, but I think that is why the bible speaks so prominently of faith. (Incomplete definition warning) Faith is belief without definitive proof. If we know something, then we don't have faith, we have knowledge. (rom 8:24 For in hope we have been saved, but hope that is seen is not hope; for who hopes for what he already sees?, heb 11: 1 Now faith is the assurance of things hoped for, the conviction of things not seen.)
I think most fanatics operate on faith, not knowledge. Knowledge needs to be concrete. Faith allows an unstable person to justify irrationality.
Some people may try to argue that all faith is irrational. I disagree. For example, the idea that the world existed before my birth is based purely on anecdotal evidence. I don't know of anyway to conclusively prove that the world and I did not pop into existence at the same instant. I accept it on faith that it did.
It's late and I'm rambling. I think I will just shut up now.