So how do we answer such a question? Philosophers and theologians throughout history have wrestled with this and what you are about to read is not original with me. But I think I can explain it better (we'll see) so here goes.
In order to answer this question we need to do a couple of things. We need to start at the beginning and we need to follow the evidence, interpreted by reason, wherever it goes. It is apparent to me, and surely to you too, that believing in a God that doesn't actually exist is about the most brainless thing a person could do. But it should also be clear that not believing in a God that does exist doesn't seem to be a very shrewd move either. We live in a binary world of true or false. Things are a certain way (God exists) or they are not (God does not exist). There is no hedging this bet. If you have ever seen Texas Hold 'Em on TV you understand that when players go "all in" they are risking every chip, every dollar, that they have. So it is with us. Perhaps this is Pascal's wager updated to the 21st century. Regardless, we, all of us, are going "all in" with the biggest cosmic bet one could ever possibly make. If we have eternal souls, then we do, and we will spend eternity in one of two places (if it turns out that the Bible actually is the Word of God). If we don't, well then, we don't and as soon as we die that's it and who cares. But whether we believe in the "God universe" or the "not God" universe, one of them surely exists. I'm not trying to put a lot of pressure on this choice but it does seem to be significant. That's why I will be very careful with the arguments. Please note that one of my arguments for the "God universe" will not be "to hedge your bets." It's either true or it's not. I think we should figure that out and act like it. Whatever that way is.
The cosmological argument has a long and illustrious history and if you want to read more about it may I suggest you get William Lane Craig's book The Cosmological Argument From Plato to Leibniz. Meanwhile, I will walk through the background to it in my own way.
The key insights are two in number and both of them involve the concept of infinity. If you get this then the rest will make perfect sense. Infinity is the idea that something has no beginning and no ending. For example, the set of all integers has no beginning (what is the smallest negative integer?) and no ending (what is the largest positive integer?) On the other hand, if something is finite (that is, not infinite) then, it has a beginning and is countable. Note that a finite series may not have an ending but it will never be infinite because you can't make something finite into something infinite by adding one more. Why? Because you can always add one more. This may seem trivial or pedantic but bear with me. This is an exercise in pure reason and it will pay off because it will make your brain bigger. Or something like that.
The first thing we notice is that it is impossible to traverse an infinite amount of time or an infinite distance. Let's pretend for a moment (philosophers call this a "thought experiment") that we have a combination space ship and time machine. It is a most powerful vessel. It can leap through time at the rate of a trillion years per second and it can travel through space at a trillion times the speed of light.
Now, here's the question. Can we, in this space ship, cross from point A to point Z if there is an infinite distance between A and Z? (Assume for a moment that an actual infinite is possible in the physical universe, which it isn't, but we'll get to that in a moment. For now, pretend.) Obviously not. For even if we traveled for trillions and trillions of miles, we wouldn't even be started on getting across an infinite distance. Why? Well because for every trillion miles we covered there remains an infinite number of miles to cross. In the same way, if point A is sometime in the past and point Z is some time in the future and an infinite number of seconds lie between A and Z, we can never get from A in the past to Z in the future.
So we know that we cannot traverse an infinite time or distance, because, by definition, an infinite number of seconds or miles is without beginning or end.
The second insight about infinity is that nothing that is material, that is, is part of the physical structure of the universe, can actually be infinite. This includes space/time. We've already seen that an infinite series of miles or seconds cannot be traversed but the fact of the matter is that it is impossible for an actual infinite to exist. Here's why. Just imagine, if you can, an infinite number of seconds (or miles or rocks or atoms or whatever). Now it's not infinite because we can always add one more second or mile rock or atom or whatever. In fact, we can add a trillion more. A trillion plus a trillion more. A trillion times a trillion more. A trillion raised to the trillionth power more. You get the point. It is impossible to have an actual physical infinite. (Space and time, or space/time, is part of the structure of the physical universe - Einstein proved that with General and Special Relativity.) Here is an easier way to understand that nothing "material," including space/time, can be infinite. It's because if something is physical, or measurable, then it can be counted. It might take a long time, but it can be counted. And if it can be counted, it isn't infinite.
This allows us to understand that the universe had a beginning. It HAD TO HAVE A BEGINNING. Why? Because if it did not, if time were infinite in the past, we could never have arrived at today. But we are here, today. Therefore, time had a beginning. Another way to think of this is to imagine that the series of seconds that terminates at this instant had to START sometime. If it never started then how could we arrive at today? If we say that it didn't have to start, then we say that the beginning of our series of seconds had no beginning (is infinite). Which, of course, is nonsense - to say that the beginning had no beginning. As we remember the law of Identity and the law of non-contradiction.
This is pretty "out there" if you've never thought about it before so I'll try to explain it in a slightly different way and perhaps that will help. This time we'll think of it in terms of causes.
Let's conduct another thought experiment. Let's assume that we have a pot of water sitting on a stove and it is boiling. Let's also assume that we want to investigate this scientifically. That is, we are engaged in the investigation of the causal chain that is producing the effect of the boiling water. So we see the water in the pan. We see the bubbles. We see the steam rising. We see the flame from the burner. We see the stove. We see the gas line. We wonder about these things. They must also have explanations. If we keep tracing things back in time we will eventually trace our chain of causes back to the beginning of the universe. But wait, you say, what if the universe, and thus our antecedent chain of causes never began? Then you are saying that the antecedent chain of causes is infinite, which is to say that it never started. But it did start because we are looking at the boiling water. To claim that an infinite regress of causes could actually terminate in an effect is to say that the antecedent chain of causes that resulted in the effect, in this case, the boiling water, never began. But it obviously did begin. Witness the boiling water.
From this, we KNOW beyond all doubt, because it is an exercise in pure reason, that the universe had a beginning. Not surprisingly, science has confirmed this and not so long ago. It started around the time of World War I with Einstein and his theory of General Relativity and continued on to include Hubble's observations in the late 20's about the red shift of star light and the background radiation found by Wilson and Penzias in the mid-60's. (For which they eventually were awarded a Nobel prize. It's a funny story in a way. They were looking for something else, quasars or something, who knows, and they kept getting this interference that they couldn't explain. They even cleaned the pigeon stuff off of the satellite dish but the "noise" wouldn't go away. The "noise" turned out to be the left over radiation (heat) signature of the "Big Bang" creation event.) See? We KNEW that the universe had a beginning from our reasoning exercise and that's exactly what science found. It's what we would expect since nothing can contravene reason. Interestingly enough, science had pretty much settled on an infinite universe for a couple thousand years. It was called the "steady state" theory by Sir Fred Hoyle (Who also "named" the Big Bang. It was a term of derision at first.) It's biggest plus was that it avoided the implication of God. For after all, if the universe was eternal, if the universe "always was," then its existence didn't have to be explained. But since the realization has slowly dawned that it did begin, cosmologists have been desperately, and to no avail, trying to explain the beginning of the universe without recourse to God. (That would be "unscientific" I guess.) Would it be uncharitable of me to mention at this point that an obscure Hebrew leader got it right thousands of years ago when he wrote: "In the beginning, God created the heavens and the earth." It took modern science a while but they finally caught up to Moses.
Now here's where things get interesting. If something begins, then something caused it to begin. This is the essential insight of the cosmological argument, which we'll explore in more detail in the next post. But for now, it goes like this. (I believe this is William Lane Craig's formulation of it.)
Major Premise: Every thing that begins has a beginner. (True by definition.)
Minor Premise: The universe began. (True by reason and confirmed empirically.)
Conclusion: The universe has a beginner. (Necessarily true. It can't possibly be false.)
Details next time. Plus a bonus. Deriving God as an exercise in pure reason.