Just give a big smile.
Tell them you smile at their ignorance. That one comment does more work than getting angry at them.
If someone says something against what you hold dear, say 'I pity your ignorance.' That is all. What can you do? You can’t change someone's wrong thinking by force. They won't take it.
In those situations, sarcasm is the best way to handle it. Your sarcasm goes like an arrow into their mind and breaks the barrier which they might have created for themselves.
Tell them, ‘I feel sorry for your ignorance’ and that is good enough.
