Being purely sure (100% confident) of something (or, likewise, assigning anything zero probability) is claiming that it is indefeasible (completely irrefutable) – literally, nothing whatsoever could possibly refute it, for the simple reason that the lower the probability (or uncertainty), the stronger the evidence must be for that. (That is, for an event with low probability, the probability of the information favoring it must be high, and of course lowering it makes the requirement even higher). In the 0% case the requirement would be infinite and so there is no amount of information that would suffice.
Claiming indefeasible knowledge equals claiming unconditional belief – the same as saying “I believe no matter what”, for the simple reason that if there is no information that could refute it, then one’s belief is independent of whatever the evidence might say and so even overwhelming evidence to the contrary wouldn’t affect it.
Such a pure unconditional belief would have to be supported by another such irrefutable belief since otherwise, it’s support would be refutable and so the belief, being independent of even that, would be unfounded.
Such a belief being founded on another such belief would simply presuppose that it is rational to have such a belief, leading back to the original problem.
An totally unfounded belief would be irrational and arbitrary, allowing anything to be chosen as such and leading to inability to favor any belief, resulting in skepticism as it’s logical conclusion.
The presupposition that it is rational could be applied to anything, hence allowing every belief and disbelief to be equally well-founded leading to skepticism as it’s logical conclusion.
Therefore, being 100% sure – that is, claiming infallible or indefeasible knowledge ( “pure dogmatism”), is completely irrational and self-refuting and contrary to true knowledge, which is probabilistic at least to some degree.