Question: "What is the Cosmological argument for the existence of God?"
Answer: The cosmological argument attempts to prove God’s existence by observing the world around us (the cosmos). It begins with what is most obvious in reality: things exist. It is then argued that the cause of those things’ existence had to be a "God-type" thing. These types of arguments go all the way back to Plato and have been used by notable philosophers and theologians ever since. Science finally caught up with theologians in the 20th century, when it was confirmed that the universe must have had a beginning. So, today, the cosmological arguments are even powerful for non-philosophers. There are two basic forms of these arguments, and the easiest way to think of them might be the "vertical" and the "horizontal." These names indicate the direction from which the causes come. In the vertical form, it is argued that every created thing is being caused right now (imagine a timeline with an arrow pointing up from the universe to God). The horizontal version shows that creation had to have a cause in the beginning (imagine that same timeline only with an arrow pointing backward to a beginning point in time).
The horizontal is a little easier to understand because it does not require much philosophizing. The basic argument is that all things that have beginnings had to have causes. The universe had a beginning; therefore, the universe had a cause. That cause, being outside the whole universe, is God. Someone might say that some things are caused by other things, but this does not solve the problem. This is because those other things had to have causes, too, and this cannot go on forever. Let's take a simple example: trees. All trees began to exist at some point (for they have not always existed). Each tree had its beginning in a seed (the "cause" of the tree). But every seed had its beginning ("cause") in another tree. There cannot be an infinite series of tree-seed-tree-seed, because no series is infinite—it cannot go on forever. All series are finite (limited) by definition. There is no such thing as an infinite number, because even the number series is limited (although you can always add one more, you are always at a finite number). If there is an end, it is not infinite. All series have two endings, actually—at the end and at the beginning (try to imagine a one-ended stick!). But if there were no first cause, the chain of causes never would have started. Therefore, there is, at the beginning at least, a first cause—one that had no beginning. This first cause is God.
The vertical form is a bit more difficult to understand, but it is more powerful because not only does it show that God had to cause the "chain of causes" in the beginning, He must still be causing things to exist right now. Again, we begin by noting that things exist. Next, while we often tend to think of existence as a property that things sort of "own"—that once something is created, existence is just part of what it is—this is not the case. Consider the triangle. We can define the nature of a triangle as "the plane figure formed by connecting three points not in a straight line by straight line segments." Notice what is not part of this definition: existence.
This definition of a triangle would hold true even if no triangles existed at all. Therefore, a triangle's nature—what it is—does not guarantee that one exists (like unicorns—we know what they are, but that does not make them exist). Because it is not part of a triangle's nature to exist, triangles must be made to exist by something else that already exists (someone must draw one on a piece of paper). The triangle is caused by something else—which also must have a cause. This cannot go on forever (no infinite series). Therefore, something that does not need to be given existence must exist to give everything else existence.
Now, apply this example to everything in the universe. Does any of it exist on its own? No. So, not only did the universe have to have a first cause to get started; it needs something to give it existence right now. The only thing that would not have to be given existence is a thing that exists as its very nature. It is existence. This something would always exist, have no cause, have no beginning, have no limit, be outside of time, and be infinite. That something is God! "The heavens declare the glory of God, and the sky above proclaims his handiwork. Day to day pours out speech, and night to night reveals knowledge" (Psalm 19:1-2).