What is it about the church that screwtape does not want the patient to see

Screwtape doesn't want the patient to see the reality of the Christian faith in the people who surround him.

At his present stage, you see, he has an idea of "Christians" in his mind which he supposes to be spiritual but which, in fact, is largely pictorial. His mind is full of togas and sandals and armour and bare legs and the mere fact that the other people in church wear modern clothes is a real—though of course an unconscious—difficulty to him. Never let it come to the surface; never let him ask what he expected them to look like. Keep everything hazy in his mind now, and you will have all eternity wherein to amuse yourself by producing in him the peculiar kind of clarity which Hell affords.