The bunker is old. Larger than it needs to be and virtually self-sufficient, built for the worst possible wars. Even before the worst came to pass, the scientists who live there rarely left. With every trip to the surface revealing only greater reasons for despair, they buried themselves deeper and deeper in their work.
Their project had been the creation of a proof-of-concept for super-soldiers. As large as possible a spectrum of ideas and techniques were collected and tried. Genetic engineering, psychological engineering, in-utero surgical modification, drug glands, lifelong 24-hour training regimes, even fringe ideas like sensory deprivation while on psychedelics and nested coming-of-age ceremonies, both designed to impart a lasting paranoia about even the most basic facts of reality.
Finally, with all avenues explored, they realized they could proceed no farther without putting their methods into practice. Producing super-soldiers was now pointless, but they were terrified at suddenly living without an explicit goal. Finally the solution came to them. The world above was chaos, cults and clans in slow decline. It needed unity, purpose and leadership. They must create hope. They must make a god.
God's genes are a human ideal, and God's brain chemically shaped from the moment of conception and has been in an induced meditation for as God has had a brain. God was born into satori, never to leave, from there to unite humanity, to lead humanity, and to reshape humanity. Soon the bunker doors will open, and a bright light will spill out.