I have recently come to the conclusion that there are cognitively functional people with a sound mind that believe that it is not possible to know anything for sure. Like, it's not possible to know if the scientists are telling the truth. We just have to take their word for it. Why not stick to the thousand year old belief system then? It has better apologists, armed with an experience of hundreds of years of demagogy and dogma regarding fending of criticism of said belief system.
That's just the excuse they use. Sadly it works on a lot of people who think it's a legitimate question when it's not, and is in fact easily disproved.
Take away their wallet, phone or medicine and then ask them if they still aren't sure. Or point a gun at their head and ask them if they're not sure a bullet will blast through their skull and kill them if they don't stop talking.
Or just tell them that means their religion is still bupkis because if we can't be sure, we can't accept it as true. Hold them to their standards.