Theists, are you immune to being decieved?

[Esquilax]

Can you define delusional for me?

And, also, could you explain how you exclude yourself from the category?

The latter question is deceptively easy: merely always allow for the possibility that you may be wrong, which atheists do by not hitching themselves to any specific god from the outset, and waiting for the evidence to fall where it may.

If you're open to correction, you aren't delusional, merely potentially, temporarily, incorrect.