From my experience dealing with secularists, ‘true believing fanatics’ is really a redundancy to them. A fanatic is, virtually by definition, someone who truly believes what they say they believe. ‘Humility’ in practice means, someone who doesn’t act on what they believe.
My response to the gent was brief:
Is your belief system a fact?
Clearly, if your belief is that all beliefs that claim to be fact must be debunked, then it is also true that this very belief that all beliefs must be debunked must be debunked.