People quit the spiritual path for a common sequence of reasons, but if you don't trigger this sequence, then disillusionment need not always happen, or to ultimately lead to abandonment of the path. When one has accepted spiritual teachings very uncritically or has abandoned intellectual evaluation of spiritual teachings, then the cornerstone of later disillusionment is set. Knowledge is gained through many different mechanisms--intellectual study, direct experience, cognition, revelation, implantation, etc. When we take all of our spiritual knowledge from only one channel, we put all of our eggs in one basket, so to speak. No matter how "into" the path we may be for a time, it is a house of cards and something will eventually make it fall, leaving us on a path that may appear either stupid, or even dangerous. So we run away and bad-mouth it. I was on such a potential path many decades ago as a youth--ran away and joined what many now say was a cult--the TM Movement of Maharishi Mahesh Yogi. I "joined" as a full-time devotee of the guru, living in an ashram and studying and working for him 24/7 for two years, 1974-76. But, I kept my intellectual doubt engaged, and that enabled me to still tell truth from bullsh*t, or at least take the diamonds which Maharishi dished out and toss away the dung that surrounded much of the rest of his organization. The knowledge was always valuable, but he was not a flawless or perfect man and he ended up surrounded by a bunch of sycophants and fools, so that MUCH of the stuff that came down was NOT truth, or even sensible, but only one side (and often a badly misinterpreted version) of things they thought the Master meant. When you follow blindly, you will eventually hit some wall, and end up with a bloody nose.
As with any spiritual practice, I advise moderation, careful scrutiny, and validation of whatever anybody tells you by a trusted person or by your own personal experiences. I've been "following" my spiritual path for 40 years now--sometimes I need to take a turn one way or the other because things change--I changed, my family and career changed, my life changes all the time. Our paths must adapt to change, otherwise we risk loosing the path entirely.
Michael