Well, when they simply will not
stop it becomes a problem.
There are some who are so convinced and/or simply brainwashed, who will not stop trying to convert you to their particular variant of VooDoo. Then I make a stand, and it can, often does get nasty.
Like you, I pick my battles carefully, I figured out at about 45 or so that I have just so much time and energy to go around.
Don't waste it fighting a loosing battle, unless you get cornered into it. Then you make sure you win convincingly. Won't change their minds, but at least they'll leave you alone.