"One man's delusion is another man's reality."
I heard a creationist tell me once before. Not that I subscribe to it but the die-hard creationists believe their world in as much as we do our own - and so to them, in some aspects of 'reality' with which we don't agree, am delusional, e.g., evolution. Taking this view will make the statement above 'true' based on their own POV. I don't think that my atheistic POV and their theistic POV are part of the same legendary elephant since atheism is the negation of theism - gods cannot exist and not exist (now nor before) at the same time. Although there are only two possible outcomes to the question 'does god exist?', it should be easy to decide which one is true and which one isn't true, right? It doesn't look like so clean given all types of variations of opinion in the spectrum . For me atheism is most probably true that I now take it for granted that it can be false. I'd say atheism is true by default - until theism is proven. (I hear the howls of the theistic defenders in the list now.)
But... I don't think the belief in a being called 'god' in itself is delusional. Delusion is too strong a word for me. But when the believer attaches many attributes and events to this 'god' such as that this 'god' is an active agent in their day to day lives, hhmm.... I believe that there are probably ET life waiting out there to be discovered but I don't think that intelligent ET have invaded our planets abducting humans (most specially American citizens) and experimenting on them.
About two weeks ago we bought this 500-piece jigsaw puzzle and started solving it but not before I took one piece and hid it in the binder on top of my PC table. As the puzzle requires a considerable effort, we only managed to finish 2/3 of it with the 1/3 lying unfinished for a week before it caught the fancy of my 4-yr old daughter again. So then we completed it except for one piece missing. I tried looking for it in the binder but it wasn't there anymore so I gave up. A few days later, my daughter found the missing piece under the rocking chair. When I asked her where she found it (it's a different piece from the one I first hid which I now presume to had been found by my wife earlier and put back into the puzzle box), my daughter said the fairy took it from the binder where she saw me looking days earlier, and the fairy moved it under the rocking chair. I told her that fairies are not real but she insisted so I did not pursue it. Is my daughter delusional? I'd think she has been deceived - that fairies exist - so I am reminding her from time to time that fairies are not real - just stories for kids. God is also just a story or but unlike fairy tales, it's one tale that most of us will be believing well into adulthood. Kids are fine for sometime believing in fairies but as for adults? I'd still say that most have been honestly deceived but if believers say their 'god' started talking to them, say to invade Iraq, that is delusional.