It can be helpful to distinguish between opinions and beliefs.  Opinions are things we learn intellectually and hold tentatively, although sometimes we are so sure they are true as to be willing to wager on them 

Beliefs, on the other hand, are things we "know" are true but never actually learned, at least in our memory.  They are obtained by being indoctrinated -- usually as children before our skeptical abilities are mature.  We are not really consciously aware of their presence -- we just assume they are true much as we assume the sofa is "there" when we sit on it without looking.

When an opinion is questioned, we ask for the evidence: when a belief is questioned we think the questioner must be nuts or perverted or lacking in some way, to question something so fundamental.

We have a strong desire (it is almost certainly an instinct evolved for group cohesion in primitive situations) to believe what the community believes and what we grew up believing.  This is powerful.  During the "rebellious" phase of young adulthood, when we are establishing our independence, aspects of the cultural environment may bring these beliefs into doubt, and even lead to their being abandoned, but not without a lot of turmoil and angst and fear and guilt.  As often as not after a few years of this sort of emotional pain the person decides "enough" and goes back to believing.  The instinct then rewards the person for doing this with joy and peace (testimonials in religious meetings show what is going on pretty well).

After this the believer is pretty much locked in and will never allow doubts again, no matter how irrational the actual belief may be shown to be.