Originally, the show was going to be called Unnatural and it was about a reporter going around researching urban legends and writing about them in his column. It had absolutely nothing to do with the Winchester brothers.
Kevin Tran was an advanced placement high school student whose life was turned upside down when he became a Prophet. Kevin translated the Leviathan tablet which led to the downfall of the leviathans, and then uncovered the secrets of closing Hell.
God, initially known to Sam and Dean as Chuck Shurley, is a powerful primordial being who created the universe and nearly everything it contains, including the low-selling Supernatural book series, which he writes under the pen name Carver Edlund.