because for one their system of belief is based on blind faith trying to argue down a system of blind faith using logic and reason is irrational, second we already know if you point out any discrepancy in their doctrine they will just tell you its because you are misinterpreting it then if you ask them to interpret it for you they are going to give you some spill they have heard regurgitated over and over again that will most likely be as logically flawed as the doctrine they are trying to protect and consistency is definitely not one of their strong points, they say god created everything but then when you asked a question like "so why do we feel the need to make weed illegal if god put it here" they will say god didn't put that there satan did i thought god created everything and everything only happens by his will, convince is a christian's biggest ally