Doesn't the universe exist in the absence of cause, and haven't we learned this in physics class? Do you not believe this as an atheist?
As far as God goes and his beginnings that's pretty deep. Most say he has existed forever and this is true in the physics of his being as true as it is that we have existed forever, both forwards and backwards. But a more down to earth answer is God once was as we are now and we may become as God now is if we live up to our potential.
@Salvation - Recall that when Jesus was resurrected and came to His disciples He ate fish with them and said "handle me and see, for a spirit hath not flesh and bone as ye see me have." God is not a spirit, He has a very real body.
@Neil - I understand what you are saying but it is false. There's a reason why God has a body and that we chose to receive a body. It's because we are more powerful with a body than simply having our spirits. We have power of the Devil and his angels because we have a body and they do not. While the physical world could not exist without the spiritual do not underestimate the power of the physical.