The novel characterizes the paradox of Gd's existence. The question is how to prove God's existence when the definition of God requires faith, and does not rely on proof outside of that. The story relies on the Babel Fish, which, for reasons completely unknown, can be inserted into the ear as a universal translating device.
"I refuse to prove that I exist," says God, "for proof denies faith, and without faith I am nothing."
"But," says Man, "the Babel fish is a dead giveaway, isn't it? It could not have evolved by chance. It proves that You exist, and so therefore, by Your own arguments, You don't. QED"
"Oh dear," says God, "I hadn't thought of that," and promptly vanishes in a puff of logic.