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.