The Earth Gods Are Coming — Kenneth Bulmer

To pave the way for galactic conquest, a united Earth had invented the robot Messiahs. These ingenious devices, dropped from space, created on each alien planet a new and convincing faith-----based on the future coming of the new men from Earth. Roy Inglis was a trouble--shooter for the "Earth Gods" operation and he was among the very few who knew that someday they would run into the synthetic equivalent of the Devil. There'd be another set of interplanetary missionaries--only these would be monsters from some anti-Earth part of the universe. But he never realized that he himself could be the first convert to the creed of the Evil Ones--and that he might never know it until it was too late for the rest of humanity!