One of the premises of your riddle is a little inaccurate.

You state, or imply, that xians "always lie". I disagree. To lie, you must intentionally tell things which you know to be false. Most xians, IMO, don't "lie", as they don't really know that they are spouting inaccuracies. Instead of lying, they are simply ignorant. They are not using intentional guile. They have not done their own critical thinking, or maybe have never even read what the Torah says. They certainly will not admit that there are any logical impossibilities.

For example, those people 500 years ago who thought the world was flat, would not be lying if they preached this knowledge to their students. However, once civilization learned the world was round, and people who knew better continued to claim it was flat for some selfish purpose.... THOSE people would be guilty of lying.

I think xians, rather than being liars, are more likely well-meaning uneducated people.