One of the greatest thinkers in Christendom who ever lived was Thomas Aquinas (1225-1274). He was a prominent scholastic philosopher in the Medieval period, where philosophers wrote very systematically.
In his treatise on the existence of God (in his work Summa Theologica), he began by pointing out the aim of the Scripture as “to teach the knowledge of God, not only as He is in Himself, but also as He is the beginning of things and their last end.”
God has implanted in us by nature the knowledge of his existence, although “in a general and confused way,” as a consequence of the Fall. As the apostle Paul says, “For his invisible attributes, namely, his eternal power and divine nature, have been clearly perceived, ever since the creation of the world, in the things that have been made. So they are without excuse” (Rom. 1:20, ESV).
One example of this is man’s desire for happiness – something known by him deeply. “For man naturally desires happiness, and what is naturally desired by man must be naturally known to him.” This is a desire for something more. And God is the ultimate source and fulfillment of this desire.
On the other hand, in demonstrating the existence of God, Aquinas distinguished two ways of demonstration. One is called a priori, which is to argue from what is prior or already true. That is, arguing through the cause. The other one is called a posteriori, which is to argue from the effect. That is, “to argue from what is prior relative only to us.”
“And from every effect the existence of its proper cause can be demonstrated, so long as its
effects are better known to us; because since every effect depends upon its cause, if the effect exists,
the cause must pre-exist. Hence the existence of God, in so far as it is not self-evident to us, can
be demonstrated from those of His effects which are known to us.”
The Five Ways
In this, he goes to lay out his famous Five Ways using a posteriori demonstrations. From the lack of space, however, let me paraphrase it:
The first one is the argument from motion. Things that are moved are moved by something else, either from infinity or from an Unmoved Mover. It’s not reasonable that it is moved through from infinite regression. Thus, it must have been moved by an Unmoved Mover – God.
The second one is the argument from the nature of the efficient cause. Things have efficient causes, either by external causes from infinity, a first efficient cause, or it is caused by itself. It’s not reasonable that a thing can have an external causes from infinity, nor being caused by itself. Thus, things must have a first efficient cause – God.
The third one is the argument from contingency (or from possibility and necessity). There are contingent things. If everything is contingent, then at some point in time everything must have ceased to exist, which is absurd because we are here. Therefore, there must be a necessary per se being – God.
The fourth one is the argument from the gradation to be found in things. There are gradations of excellences in this world. One is nobler than the other. Some things are better than others. Therefore, these must be a perfect being whom excellences proceed from – God.
The last one is the argument from the governance of the world. There’s order or ends (telos) in things in the world. “It is clear that it is not by chance, but by design, that they attain the end.” Just like an arrow being directed by an archer. Thus, there must be an intelligent being that directs all natural things that are ordered to an end – God.
From this, the existence of God, although known by our desires for fulfillment and happiness, can be rationally demonstrated.
 Or else today would have not arrived.
 Or else it would be question-begging.