Mahayana Buddhists have a saying to the effect that as one gets closer to enlightenment (or Pure Land, Nibbana,etc.) all dharmas go away. Essentially it means the various practises and beliefs gradually become unneeded and unuseful as one develops knowing. This does not mean what is true now is not true later, but more that what once worked will not work. Imagine you are just starting to life weights. You wouldn't begin by bench-pressing 300lbs. You would use repetitions of some weight much lighter and gradually increase. There would be a point that very light weights wouldn't be of as much use as heavier ones, as you built muscle mass.
People sometimes outgrow their "faith.", and sometimes they find they can't have "faith" in anything without some reason. A baby, as it learns speech, accepts the words of its mother, but even then it has to find out things for itself. This is not mistrust so much as it is the Brain OS loading and doing it's thing. Later on, though, I think most people do tend to be wary of taking anything on word alone, with spirituality perhaps being the biggest exception, as it is easy to take someone's word and put it on the backburner, or else perform some token thought or service towards it during the day (rosary, meditation, prayer, putting money in the McDonald House box on lunch). These activities anchor and maintain one's "faith", to a degree. Somewhere along the way, however, the urge to question, seems to get lost.