6 ideas
Someone who has heard only the arguments for his own position cannot really judge it, because he doesn't know the grounds of the opposing view well enough to refute them. His reasons may be good and still convince no one, since he cannot show why the alternative fails. Without that, he has no rational basis for preferring either side.
Hearing opposing arguments from your own teachers, framed and pre-answered by them, is not enough. You must hear them from people who actually believe them, defend them in earnest, and put them in their strongest form. Only this exposes you to the part of the difficulty your side has not yet dealt with.
A true belief held without ever being contested becomes dead dogma: its words are repeated, but its meaning no longer shapes feeling or conduct. Active challenge keeps the reasons and the practical force of a belief vivid in the believer's mind. When challenge stops, even correct doctrines lose their power to guide action.
To suppress an opinion because you are sure it is false is to treat your certainty as absolute. History shows ages, churches and individuals confidently holding views later seen as wrong. Being confident enough to act on a belief is legitimate only because the belief stays open to refutation, and suppression removes exactly that condition.
Conflicting doctrines often each hold part of the truth rather than one being true and the other false. The dissenting minority opinion often supplies the part the prevailing view has neglected. Suppressing it therefore removes a necessary correction, even when the dissenter is mostly wrong.
Socrates was executed for impiety and corrupting youth, and Jesus was crucified for blasphemy. In both cases sincere, respected judges acted in good conscience by the best moral standards of their day. The examples show that being conscientious and reputable offers no protection against persecuting the truth when you suppress opinions you think dangerous.