Beware the "True believer"

I always feel a little worried when I meet anyone who has complete and truly unshakeable belief in anything.
I’m not against faith as such, I hasten to declare. Millions among us have faith in a divine authority. Christians, Muslims, Bhuddists (more or less), Hindus and many more divisions, offer us a pattern for life and hope for the greater future.
Aussie rules, Soccer and Rugby players and supporters have and need great belief in their teams, we (mostly) believe in our nation, our system of government, our politicians, our friends and our family, and there’s nothing wrong with that.
It isn’t the institutions that worry me – it’s the “true believers,” those whose faith forbids thought or question.
Religion fills a widespread need, but differences between the divisions have festered throughout history. Pride in nation and government is good, but reasonable people leave room for reservation in their enthusiasm for their own and others’ systems.
I admit that I’ve sometimes felt fear in facing some football fans, or chanting crowds in noisy demonstrations.
I don’t know of any sure way to defeat fanaticism of any form, but I reckon we’d do better in this world if more were to “use the brains we were born with”.