I should have elaborated.
- they will view it as an attack upon them
- they will be forced to shun you and think of you as an apostate
- you can kiss any inheritance good-bye
- you'll be unable to have interactions with them, so as to enlighten them, because the well will have been poisoned
Silence/"discouraged"/"stumbled" won't bring these negative consequences, in many cases.
You can maintain the benefit of the doubt by keeping the truth to yourself. By telling them you don't believe, you switch off their minds, allowing them to dismiss you.