Some of the oldest and more widely used arguments for the existence of God are the cosmological arguments. The four to be addressed here are those which Thomas Aquinas listed in his Summa Theologica.
The Argument from Motion
Thomas Aquinas put this argument thus:
It is certain, and evident to our senses that in the world some things are in motion. Now whatever is in motion is put in motion by another, for nothing can be in motion except it is in potentiality to that towards which it is in motion; whereas a thing moves inasmuch as it is in act. For motion is nothing else than the reduction of something from potentiality to actuality. But nothing can be reduced from potentiality to actuality, except by something in a state of actuality.
Then he basically goes on to explain that because of this, everything in motion must have been caused by something already in a state of actuality, and you could keep going backwards to infinity. Because he believes that going backwards to infinity is impossible and illogical, he concludes that there must be a first, unmoved mover.
Now, that the universe cannot be infinite is evident. To demonstrate, we can define the universe as the sum of its parts, which can be said of nearly anything physical. Now, every part of the universe that we have observed is temporal. Even the sun, for example, will one day die. Therefore, if the universe is the sum of its parts, and if every part of the universe has been observed to be temporal, then the universe cannot be infinite. Therefore, there must be something else that has set in the universe in motion. If one believes in a multiverse, this still holds true, since a multiverse made up of temporal universes must itself be temporal.
There must be a something or someone that has never needed to be moved, but is itself the unmoved mover. Thomas Aquinas stated that this must be God.
The Argument from the Nature of the Efficient Cause
Thomas Aquinas’ second argument is thus:
There is no case known (neither is it, indeed, possible) in which a thing is found to be the efficient cause of itself; for so it would prior to itself, which is impossible. Now in efficient causes it is not possible to go on to infinity, because in all efficient causes following in order, the first is the cause of the intermediate cause, and the intermediate is the cause of the ultimate cause, whether the intermediate cause be several, or only one.
Basically, the point he’s trying to get to is that there must be a first, uncaused cause, since every effect must have a cause, so there must necessarily be a first cause, that is itself uncaused. If there were not a first cause, he explains, none of the effects would have followed. This first efficient cause, he argues, is God.
The Argument from Possibility and Necessity
Next, Thomas Aquinas makes a somewhat modal argument, that is, an argument discussing things in terms of modes, such possibility and necessity. Here’s the beginning of his argument:
We find in nature things that are possible to be and not to be, since they are found to be generated, and to corrupt, and consequently, they are possible to be and not to be. But it is impossible for these always to exist, for that which is possible not to be at some point is not. Therefore, if everything is possible not to be, then at one time there could have been nothing in existence. Now if this were true, even now there would be nothing in existence, because that which does not exist only begins by something already existing.
To summarize the rest of the argument, he makes the point that because things do exist, not everything can be possible to be or not to be. Therefore, there must be a necessary cause that already existed that put everything which is possible but not necessary into existence. This necessary cause is said to be God.
The Argument from Gradation
This fourth argument of Thomas Aquinas for the existence of God is similar to the rest in that he says that there must have been someone from which certain things flow. In this case, it is “being, goodness, and every other perfection.” The analogy he uses is that all heat comes from a source, for example, fire. Of course, for fire to transfer heat, it must itself be hotter than the things to which the heat is transferred. Similarly, goodness must have a “maximum” as Aquinas puts it. This maximum must be the source of all goodness. According to Aquinas, this maximum is God.
In my opinion, the first two arguments are the best, since they are the easiest to understand and demonstrate, making them the most practical. Furthermore, all of these arguments may be used as ways to consider God or as ways to refer to Him. For example, He may be called the Unmoved Mover, or the First Mover, and the First Efficient Cause.