It would only make sense that the only character on "The Simpsons" worth having five fingers would be the most omnipotent of figures. First appearing in the Season 4 episode, "Homer the Heretic," God's design is the only one to consistently be shown with five fingers. He usually makes appearances when someone questions religion or his abilities. These include bringing the sun out to ruin both Homer and Barney's plowing businesses after Homer brags that not even God can stop two best friends when they're working together, aiding Ned Flanders when his son Todd is floating down a river, and stopping a flood going through Springfield when Reverend Lovejoy asks for forgiveness. While given an extra finger, one feature we almost never see on God in the show is a face, which is only seen on a few brief occasions. Additionally, Jesus is occasionally shown to have five fingers as well, but not to the extent that we see in God's design.
Given how difficult hands can be to draw in animation, amongst other factors, artists have generally always stuck with the four-fingered design for their characters (via The A.V. Club). According to the commentary track on "The Complete Fourth Season" DVD set for "Homer the Heretic," God's five-finger design was a deliberate choice to help make him more unique and allow him to stand out as a divine figure. Even if it is a small detail, it nevertheless adds to the already-lively world of "The Simpsons" that has helped make it such a beloved show.
