Babies come from mommies and daddies, cars come from builders, and trees come from acorns. And though adults may simply rest content with that explanation, little children inevitably ask, "Where do the parents, builders and acorns come from?" And so we find that everything participates in a "Great Chain of Being" which takes us further and further into the past until we get to the Big Bang itself. In the normal course of events, absolutely nothing in Nature is unhooked from that chain. Everything in this universe is caused by something else in this universe which is caused by something else in this universe and so on and so on and scoobie doobie doobie. Our awareness of this is so fundamental that when something does break that Great Chain of Being (as, for instance, the miracle of the loaves and fishes does in John 6:1-14), we have to find an explanation for it by either saying, as Christians do, that the God who is beyond Nature miraculously added some links to the Chain, or else we must say, as skeptics do, that it has some sort of natural cause (i.e., people sharing lunches, or a big lie by the apostles who were, so to speak, yanking our Great Chain of Being). The one thing nobody believes in is what philosopher Peter Kreeft calls the "Pop Theory" —that things like loaves and fishes just pop into existence for no reason at all. They must have a cause, either natural (i.e., bakers and fish eggs) or supernatural (i.e. direct creation by God the Creator). Nothing in this world can cause itself to exist. Every created thing relies on something ahead of it to pull it into being, just as a boxcar relies on the car ahead of it to pull it uphill.
Very well then, if everything in nature is like a big train going uphill, we have to ask, "What is the Engine?" We can't say that there is some break in the chain—that some natural thing just popped itself into existence 13.5 billion years ago just as we can't say that loaves and fishes popped themselves into existence 2,000 years ago. All natural things are contingent—they depend on something else in order to begin, change or move. The great pagan thinker Aristotle put it simply: “Everything that is in motion must be moved by something.”
Now we can have a very long train of contingent causes—just so they aren’t all contingent. Why? Because we can’t say that there is an infinite train of boxcars going up an infinite hill without an engine, since the nature of every car in the train—from quark to quasar—is that it requires something else to cause, change, or move it. If everything that makes up the universe is contingent, then the universe made of all that contingent stuff is contingent too. At the end of the day, a purely contingent universe still has to borrow its existence from Something that isn’t contingent.
Seems simple and obvious enough, and it is. Which means that those bent on avoiding You Know Who work hard at overlooking the simple and obvious. Of which more next time.