As for the bolded part: If you had been delusional, you couldn't have critically examined your delusions and adjusted your thinking. You would have not had the option of adjusting your beliefs based on your evolving understanding. Delusional people also can't deconvert out of their delusions, like many people deconvert out of religious beliefs. The delusions are overwhelming and will override the delusional person's rational mind. The process how delusional mind develops, is a one-way street, like most mental illnesses are: the condition that makes people delusional typically only deteriorates and the delusions become worse and worse over time. Religious beliefs on the other hand have a wide variety of ways with how they develop, some people become more religious by age, some become less so. There is actually strikingly little in common in the nature of delusions and religious beliefs on a closer examination.
The topic makes me think of the name of the famous pro-atheist book "God Delusion" by Richard Dawkins. The name of the book is not an accident, it follows the anti-religious tradition of rhetorically equating religion with mental illness. Basically the name could be "Only crazy people can believe in God". It is typical of all bigotry to be rooted in ignorance, and the Dawkins-style "religious beliefs are delusions" - bigotry is not an exception, it's classic ignorant bigotry in the sense that it's based on (either genuine, or intentionally chosen rhetorical) ignorance about the nature of delusions and the nature of religious beliefs.
Granted, there are superficial overlappings when it comes to how strong mysticism and some mental illnesses appear to the outside world. Mystics can come off as little bit nuts or at least wierd. But, I think part of the problem with that observation is the silly assumption that our views on God are supposed to develop in the same purist rational fashion as our views on atomic theory do. There are other areas in life that are driven by something else than reason, such as artistic inspiration or falling in love. A person who has fallen in love is also a bit "crazy" and so is a person who is in the midst of a deep artistic inspiration. But, people don't make as much of a problem out of those phenomenons because our culture is more used to people who fall in love, or express artistic eccentricity, than it is used to religious mysticism (in some other cultures, religious wierdness was much more acceptable).
Healthy human nature as a whole is not a cold, hard, rational, facts - examining computer. There are highly extra-rational elements to human nature (artistic inspiration, falling in love f.e.). In my opinion, there is no reason to expect that spirituality or beliefs should be made of cold analysis either.