Why do juries not believe in God?

It’s a peculiar thing, but when juries, even the most evangelically oriented with a southern Bible-belt God-fearing judge, hear a defendant say “God told me to do it,” as has happened; those juries uniformly disregard the belief of the defendant and convict them. We have an insanity defense, but in a country that wants the Ten Commandments on the walls of buildings and prayer in the schools, there is no “God told me to do it” defense even though in the Bible it’s a common occurrence. So do people really have faith in their faith?