When is a Religious Belief Delusional?

[Fake Messiah]

I don't consider that having a delusion necessarily implies any kind of mental problem. It's more to do with how and why it arises. If you're surrounded by people who think the same way, it's much less likely to suggest there's anything to worry about than if you hold such beliefs in isolation.

Some countries are completely packed with theists, mostly all of the same religion. It's ridiculous, in my opinion, to suggest they are all mentally ill. They have been indoctrinated, and they have their beliefs reinforced every single day.

Sure, but there are such things as driving someone crazy. Brian Baker, who was an ex Christian preacher, explained it best when he said how he healed himself from Christianity:

In effect I had removed myself from the ‘Christian bubble’ in which I had resided for so many years – at last I was free to look into my beliefs from the outside. As a totally committed Christian within the ‘bubble’ it is almost impossible to see or to examine any belief or evidence ‘outside.’ This is the main reason why Christians are not open to question their own faith. Fear is the other major factor why believers are reluctant or refuse to consider anything which may question their faith. Simply they are convinced that any contrary teaching or evidence which is not Bible based is ‘satanic’ in origin.