St. Thomas Aquinas offers five arguments to prove the existence of God. These are motion, the efficient cause, possibility and necessity, the gradation of things and the governance of the world.
First of all, everything in the world is moving and changing, however there must be some outer force to make a thing move or change. Otherwise, it is only potentially moving, and an act is needed to turn the thing from potentiality to actuality. One thing cannot be the mover and moved at the same time, so there must be the first mover not liable to any other outer forces. This first mover is God.
Secondly, as Aquinas underlines, each phenomenon in the world has its cause. The causes are revealed by their effects, but the list of causes cannot be endless. Thus, there must be some first efficient cause to start the chain of all the next causes, and this first efficient cause if God.
Thirdly, each thing in nature has a potential to be or not to be, however there must be someone or something to turn the thing from non-existence to existence. “It is impossible to go on to infinity in necessary things which have their necessity caused by another” (Aquinas). So, there must be something existing by its own necessity, and this is God.
Further on, Aquinas explains that there is the gradation of things. It means that all the qualities found in things are expressed to a certain extent and they are compared to a certain ideal, or maximum. It is stressed that there must be some ultimate ideal of everything good, beautiful and virtuous, and this maximum is nothing but God.
Finally, Aquinas demonstrates how the governance of the world proves the existence of God. There are many natural bodies lacking intelligence but working for a definite end. It is concluded that they must be guided by some intelligent entity to be directed to an end, and this being is God.