When is a Religious Belief Delusional?

[Joods]

From the tail end of another thread…

Whether or not a theist's perceptions of God represent a "delusion" (in the sense of being a firmly maintained yet irrational belief) was the conversation I was interested in having.

So I say, lets have that conversation.

Let me first say that, as a Christian, this is very difficult conversation to not take personally. ‘Delusion’ has a very negative connotation, even when used colloquially. For many Christians, and I suppose believers in general, their religious convictions are central to their identity and serve as the core around which their values and principles develop. Having it maligned is perceived as dismissive and disrespectful.

At the same time, sincere and thoughtful believers are very much aware that there are seriously disturbed people who truly believe supernatural entities have directed them to do dangerous and/or heinous acts (a mother drowning her three children in a bathtub because she thought they were demons, comes to mind). For a non-believer, to lump mainstream Christians in with people who are clearly mentally ill and failing to acknowledge various safe guards (that are by no means infallible) built into religious traditions, is at best IMHO thoughtless and at worst offensive and provocative.

So for the purposes of this conversation, I will assume that ‘delusion’ is being used moderately by atheists, not indicative of actual mental illness, but rather as something that hinders reasoning. As such, what criteria must be met for a belief to qualify as a delusion? Some I have heard some of the following proposed and with which I agree:

- Unquestionable certainty.
- Resistance to scrutiny.
- Contrary to what is demonstrably false.

bold mine. 
Clarification needed here. If it's to be considered a delusion, wouldn't the thinking have to be contrary to what is demonstrably true?

That said, the devil is in the details. My initial question to atheists is this. How do you justify the claim that all religious beliefs, or those most commonly held, satisfy the above criteria (or others you may propose that are not listed)?

For me personally, I justify the claims of believers fitting the above criteria (with the third one being corrected) because 
1. There is no certainty or guarantee that your beliefs are true and correct, regardless of what the bible says.
2. Whenever any religion is up for scrutiny and doesn't pass the litmus test, believers go all on the defensive and immediately close their minds to the facts that science has provided, which can't be ignored by those possessing logic and reason. 
3. Comes back to the second point. If things are demonstrably true, as has been tested by the Scientific Method, believers still refuse to come around to seeing the logic and reason for the non-existence of a god or gods. 

So I pose the following question to you: 

If you can show me why any of the Greek gods are false, then please do. I'll be happy to use your own reasons and explanations as to why the Abrahamic god you follow is also false. [/quote]