Earlier quoted context omitted.
I would listen to the person's arguments and say what I don't like about those arguments. I find the symmetrism you represent ridiculous. How do we know the Earth isn't flat? Maybe flat-earthers are right? How do we know if santa claus exist? Well, we gather data and apply reasoning to that data... https://en.wikipedia.org/wiki/Epistemology
Just like you listen to the persons arguments and engage with them when someone calls you and says "Hello I am federal tax Marshall john Smith, and there is a warrant for your arrest. Or you can buy gift cards and send money to us to settle your debt." I'm certain you spend a while evaluating and debating those arguments.
I had a Jehovah's witness visit me once, and unlike most people I did talk to her. She made some arguments, I counter-argued that I don't believe things without a proof, she asked me if I believe that air exists, and she followed my confirmation with the statement "but you can't see it" - to which I responded that I actually can see both the air directly (just like you can see foil laminate if you stack it into multiple layers) as well as indirectly through various experiments. She had no response to that, other than handing me a flyer, which I accepted.