When you speak of "God" are you talking about the christian God? Or the islamic God? Or perhaps the jewish God? (If they're the same, why would they have ordered the writing of different books?) Or even some of the countless gods that humankind has created and talked about? There is nothing such as "THE definition of God". There are many definitions.
About Stephen Hawking, we don't actually KNOW if the Universe has had an origin. We have the big bang hypothesis, and even the big bang may be the consequence of something. Perhaps (and most probably to me) the Universe has always existed, though it may change shape and properties.
Second, I think that "impossible" is a very strong word to use, since we may never know 100% if something is or isn't possible. Maybe in different parts of the Universe, or even somewhere out of this Universe, something that looks impossible to us may be possible. There is the hypothesis that our whole Universe may be only a black hole inside of something else. Maybe this goes on forever...
So, returning to gods, I call them Mythology. Just like the science about the origins of the Universe: things we are never going to know.
Daoist philosophy talks about the Dao, which may be translated simply as Nature or Way (as in "the way things are"). The Universe exists by itself, and creates things because it's always in movement ("The movement of the Dao is the return" says the Dao De Jing). I think that's what Spinoza had in mind.