I don't think that being delusional is quite the same as being crazy, although many crazy people are delusional. I think a delusion is a fixed false belief without evidentiary support, that is not subject to rational argument. When someone says to me, "I believe this on faith, regardless of the evidence, and there is no evidence that would cause me to give up my faith," that says delusion to me. A good example would be a Young Earth Creationist. A sign of a delusion is that no amount of evidence or logic can dislodge it. I don't think intelligence is a very reliable protection against delusion, if a person is willing to set their intelligence aside, so to speak, and go on "faith," meaning here belief aside from or even despite the evidence.