'A delusion, in everyday language, is a fixed belief that is either false, fanciful, or derived from deception'
This is the Wiki definition for delusion. Now I personally would say that a lot of the aspects of religion do count as fanciful (rise from the dead anyway, virgin birth, just from christianity we can see a few bits which are a bit )
'False' well a lot of religions come down quite hard on 'false gods' however I also think you have to have faith to be religious and a central tenet of faith is that you have to believe something without firm evidence and in that case there is always the chance you are wrong.
'Derived from deception' well I would say, that centuries ago, many aspects of science and demonstrations thereof would be considered 'miracles'. So many religions may have been founded on misunderstandings or deceptions (however motivated). Doesn't mean they are a bad thing. The tooth fairy probably isn't a bad thing, it helps children deal with a transitional process.
I am a secularist. I have absolutely no problem with people who are religious. I will support your right to worship whatever you like. I just don't want to have it in my life or that of my kids (until they are old enough to make that decision for themselves).
I think that privately you can think whatever you like, YABU to think the religious are deluded and under the strict definitions of the term, you are probably right.
However it doesn't matter what you think of everyone else ATEOTD, all that matters is your behaviour.