The greatest effect on belief is our conduct. A young person does not fall away from the faith because he read a book and thought the thing out. Faith is weakened when we act against moral truth. As soon as I commit a sin then I must repent which is to see the truth of the moral law, or I must use my mind to justify my sin. The more attached I am to a sin then the less clearly will I be able to see the truth. It is often easier to change the rules to fit my life. Then I don't have to feel guilty.