To really sum everything up is quite simple.
According to the strong anthropic principle, there are either many different universes or many different regions of a single universe, each with its own initial configuration and, perhaps, with its own set of laws of science. In most of these universes the conditions would not be right for the development of complicated organisms; only in the few universes that are like ours would intelligent beings develop and ask the question: "Why is the universe the way we see it?" The answer is then simple: If it had been different, we would not be here!
There are something like ten million million million million million million million million million million million million million million (1 with eighty zeroes after it) particles in the region of the universe that we can observe. Where did they all come from? The answer is that, in quantum theory, particles can be created out of energy in the form of particle/antiparticle parts. But that just raises the question of where the energy came from. The answer is that the total energy of the universe is exactly zero. The matter in the universe is made out of positive energy. However, the matter is all attracting itself by gravity. Two pieces of matter that are close to each other have less energy than the same two pieces a long way apart, because you have to expend energy to separate them against the gravitational force that is pulling them together. Thus in a sense, the gravitational field has negative energy. In the case of a universe that is approximately uniform in space, one can show that this negative gravitational energy exactly cancels the positive energy represented by the matter. So the total energy of the universe is zero.
Now twice zero is also zero. Thus the universe can double the amount of positive matter energy and also double the negative gravitational energy without violation of the conservation of energy. One could say: "The boundary condition of the universe is that it has no boundary." The universe would be completely self-contained and not affected by anything outside itself. It would neither be created nor destroyed. It would just BE.
The idea that space and time may form a closed surface without boundary also has profound implications for the role of God in the affairs of the universe. With the success of scientific theories in describing events, most people have come to believe that God allows the universe to evolve according to a set of laws and does not intervene in the universe to break these laws. However, the laws do not tell us what the universe should have looked like when it started - it would still be up to God to wind up the clockwork and choose how to start it off. So long as the universe had a beginning, we could suppose it had a creator. But if the universe is really completely self-contained, having no boundaries or edge, it would have neither beginning nor end: it would simply be. Like the south pole on the earth. What is south of the south pole? When you understand what I am saying, then as yourself: What place, then, for a creator?
So when people ask if a God created the universe, I tell them that the question itself makes no sense. Time didn't exist before the big bang, so there is no time for God to make a universe.
It's like asking for directions to the edge of the earth. In early history, the answer would simply be travel in any direction and you will eventually get there. But eventually one person came along and asked for proof and found everything about the earth having an edge was wrong. The earth is a sphere. It doesn't have an edge, so looking for it is a futile exercise.
We are each free to believe what we want, yet it is my view that is the only one that has evidence. The one that is always the simplest explanation:
This leads me to a profound realization. There is probably no heaven or hell, and no afterlife either. We have this one life to appreciate the grand design of the universe, and for that, I am extremely grateful.