Without knowing for absolutely sure that there isn't a God- you can't be delusional. Delusions are beliefs in things that couldn't possibly be true. If science could explain absolutely everything and prove that all religions are total tosh then, yes- we'd reduce the bible, the Qur'an, the Torah, the Tripitaka etc etc. to the level of fairy stories and call people delussional for believing in them as factual.
Honestly, I probably do think that total belief in some of the religious scriptures is delusional. For example, people who believe we originated from Adam and Eve rather than monkeys. There's clear proof we originated from monkeys. I think it's delusional to start coming up with conspiracy theories to account for dinosaur bones because they mess with religious ideas. I've actually heard that some people claim they were put here by the devil to mislead us! Again- science can prove how old they are, what the earth was like back then. It seems foolish to me to ignore that. I personally believe stories in the bible for instance are just that- stories that maybe have some moral lesson but aren't necessarily historically factual.
But, there's a difference isn't there? Spirituality in general is some vague belief, suspicion, hope even that there could be some superior being out there. I don't think we know enough to be able to prove or disprove that. Atheists will argue that the burden of proof lies with the religious folk to provide the goods but, I don't even know if they could prove or disprove the existence of God. If God is real, they are surely superior to us. If they don't want to be found, they won't be! It's one of those impossible conundrums.
However, absolute belief in an orthodox religion seems more shaky to me. Quite often, they were largely written up by humans- very likely with their own agenda. So for example- are extremists delusional? I imagine most people would say yes. They are likely so isolated and vulnerable and angry that they fall prey to other people's motives that convince them that their God wants them to go out and kill all the infidels. Their belief is seen as a distortion of the 'true' religion. Really though- it's another interpretation. Like all religion seems to be. We think God meant this when he/she/it or, their prophet said this... as far as we can remember. Another problem if the holy scriptures weren't written by God themself and at the same time them or their prophet was around.
You've got to wonder why God wants it all to be so ambiguous. Probably hundreds of different Gods and religions to choose from. What's the point in that? Especially if you get into trouble for picking the wrong one! Imagine Christianity got it right. What about the poor sod who got born into a strict Muslim, Hindu, Sikh family? How were they to know? Or- vice versa. Maybe we're all wrong. Maybe an entirely different God made all this and we don't even know about them!
Plus, I suppose there's individual experience. That's probably the hardest to shake because it's difficult to pin down. A feeling that there's something more to life. Maybe that is delussional but, that can feel more genuine than a whole load of science.
