Compare with "heretic". It basically means, "You used to agree with us that we had the one and only truth and we were the most holiest of the holy. Now you say it's BS. Therefore, you are evil and God will destroy you." Historically as far as I know this logic only existed in intolerant monotheistic societies, which tended to be effective militarily. A similar intolerant mindset where critical thought is hindered is brought about by patriotic indoctrination.
I thought this Wikipedia contrast was noteworthy:
"One of possible reasons for this renunciation is loss of faith, another is the alleged failure of religious indoctrination and/or brainwashing."
Blind faith is the result of indoctrination and/or a lack of sincerity. I think all religious/spiritual/political groups that claim some kind of special authority to interpret truth are basically led by mentally ill people who are intelligent enough to persuade regular folk that they are divinely inspired to whatever degree, but who lack the ability or desire to see themselves objectively or think critically about their own beliefs. This works out great because regular folk would rather not have to think too hard and their reasoning is, "Even though I can't explain x, I know my bishop/circuit overseer/imam/party leader will have a better explanation that passes the critical thinking test." Until you actually follow through and decide to dedicate the real mental and emotional effort required to sift fact from fiction and you realize how much BS there was at the core. Often when the indoctrination fails, an "apostate" still has not learned how to critically examine themselves and therefore ends up in another group that is another side of the same coin. Generally the reasons are more emotional than intellectual.