Why are you chasing the idea of the existence of a God?

[Angrboda]

The concepts upon which religions are based -are- the specific doctrinal claims, not some vague spirit-this spirit-that.  In any case, self-indoctrination isn't a mysterious or unknown quantity..so asking how religions get started if religion is a case of indoctrination (as opposed to our innate tendency towards magical thought) poses no problem whatsoever.  Case in point...spreading a religion is one of the most effective tools of self indoctrination.  Sinking a persons credibility of interaction into the ideology and providing opportunity for them to repeat, reaffirm, and rework their arguments with others to be more internally persuasive to themselves.

Faith seeks confirmation, and strengthens itself (internally and externally) through communal assent.

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Side note....the broad strokes of why religious movements follow a historic trend of interaction presents itself above as well.  Why it begins as a bunch of crackpots pleading for acceptance within the majority as an equally legitimate viewpoint..the need of assent, affirmation, or toleration...then becoming aggressive in it's outreach, and finally attempting to silence or delegitimize any remaining (or newly formed) dissent.  

The OP is a textbook example.

Given that the God helmet provokes spiritual experiences by physical manipulation of the brain without reference to doctrine, I'm not sure you're right here. Your claim seems to trend toward something like the Sapir-Whorf theory of language for spiritual experiences. At best, your contention is an unproven assertion. Given that children appear to spontaneously invent the concept of God, I think you've failed to explain even the given counter-example. Additionally, I think there's a clear distinction between a concept and a doctrine. A doctrine is a belief about the concept. I'm not sure the idea that the two are the same even makes sense. At minimum, I'd like to know more about what you mean by this. Hopefully you're not claiming that without a specific doctrine one is incapable of having a loose and amorphous concept of a thing.