The DeHaan Lens for Hawking and Mlodinow's "The Grand Design"

If something is good but unique and unlikely, that might make it "necessary to invoke God" to explain it.

Our universe has a delicate and unlikely balance of forces to make it work: for stars and supernovas, elements and planets. If the forces were slightly stronger or weaker, there would be no stars, or perhaps all stars would explode; perhaps all elements would be radioactive. Maybe we do need "God" to explain the universe.

Astronomers can see there must have been some kind of Big Bang long ago. Newton and Einstein cannot deal with the earliest moments, when the size would have been zero and the density would have been infinite.

Quantum physics does deal with such tiny things, and can say the universe had a minimum starting size. So the Big Bang must have been a quantum event.

A quantum event is not just one thing: it is a "wave" of probabilities that all are tried at once. So our quantum Big Bang is just following one of the possible situations, and not necessarily the most likely one.

If life were impossible, then we would not be here asking why the universe cannot support life. Since we are here, we must be in a universe that has the right balance of forces. We live in one of the unlikely possibilities, but since every possibility gets a "turn", our universe was bound to be one of them.

So Hawking and Mlodinow can justify saying "It is not necessary to invoke God to" explain this universe.