It depends whether they are wrong as to if you're able to say it. If it is something liek the existence of God which cannot be proven, to say they are wrong would be just idiotic. But there's also to way you tell them. Just being straight up and saying "you're wrong" means nothing without proof. So giving your knowedge on why it is wrong, then gives them the evidence they need to change their mind. If they still believe after that, well that's entirely up to them. You know what the real answer is and can be happy in that. I tend not to tell people they are wrong unless I know definitely, no doubt about it, that I know the real answer and have an explanation why. Otherwise, I just treat it as another opinion and respect it.
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