If I gave you a pair of blue glasses you could honestly claim that the world was many shades of blue - even if you previously had experience of its full spectrum. If you wore the glasses long enough or believed in some property of the glasses themselves (they reveal the inner truth of the scene - its essential 'blueness') you would be quite able to argue that the truth about how the world looked was indeed blue - and the reality would, for you be true, the world would be blue.
Faith in a concept or ideology works in the same way. The glasses are the concept that colors the world. Communism, religion, liberalism, anything that casts the world in a certain hue and is in and of itself believed will cause reality to accord. When I was an active believer God genuinely spoke to me, I had spiritual experiences, I enjoyed some moments of religious bliss and awe that were breathtaking. Through my eyes the world was genuinely playing out the battle between good and evil. My intelligence, just as surely as my eyes and other faculties are mediums and tools for perceiving the world.
The more I 'know' the more I am able to describe and compartmentalize the experiences around me but it doesn't necessarily follow that it will enable me to recognize that the 'truth' revealed by the glasses isn't superior or grounded in a platonic , objective truth.
Ultimately I need to doubt the glasses themselves, for me it was the disparity between the claims and the results, the places which weren't 'blue' even though I wore the 'glasses'. For me that was essentially the lack of real divine power being manifest via such things as healings. Its no good trying to show a believer that the world isn't blue - because to them it really is - no amount of intelligence can negate that single idea. To exit a bad / untrue idea the doubt must be in the filter itself.
