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"To one who has faith, no explanation is necessary. To one without faith, no explanation is possible."
St. Thomas Aquinas is one of the most influential Catholic theologians in history. Many of his spiritual quotes continue to provide motivation and inspiration to religious and spiritual people around the world. Most of these quotes come from his massive work, the "Summa Theologica." Considered by many to be Aquinas' masterpiece, this is where he laid out the five ways by which he believed the existence of God could be proven.
These arguments can be traced back to ideas about God in Aristotelian philosophy, which in turn have influenced religious and philosophical thinking for over two thousand years. The knowledge of these arguments and their history is important for anyone who wishes to have an understanding of philosophical and religious thought up to the present day. Whether you personally find Aquinas' arguments convincing or not likely would not trouble this Saint too much. He is famously quoted as saying, "To one who has faith, no explanation is necessary. To one without faith, no explanation is possible."
Aquinas' Five Ways are presented here in a simplified manner, in order to make them easier to comprehend. This will also hopefully act as an aid if you ever wish to read these arguments in their original form in the "Summa Theologica."
1) The Argument of the Unmoved Mover
The Argument of the Unmoved Mover states that in the world, some things are in motion. Everything that is in motion has been put in motion by another. One thing cannot be both mover and moved, because it is not possible that one thing is at once both actual and potential in the same respect. Aquinas likens this to something that is actually hot. It cannot at once be potentially hot because it already is hot; it can only be potentially cold. Therefore, everything must be put in motion by another. However, this cannot go on to infinity, because if there were no first mover than there would be no other movers. There must be a first mover, and this mover is understood as God.
2) The Argument of the First Cause
The Argument of the First Cause is similar to the Argument of the Unmoved Mover. All things in the world have an efficient cause. Nothing can be the efficient cause of itself because it would have existed prior to itself. It is not possible for this causal chain to go on infinitely because if there were no first cause there would be no effect and no subsequent causes. Therefore, there needs to be a first efficient cause, and this is God.
3) The Argument of Contingency
The Argument of Contingency argues that it is possible for things to be and not to be. If it is possible for everything not to be, then at one time there could have been nothing in existence. If this was true then there would still be nothing in existence, because the nonexistent only begin to exist by something else existing. Therefore, if at one time there was nothing in existence then it would be impossible for anything to ever begin existing. Things do exist, and so, there must be something which necessarily exists, which is eternal. This is God.
4) The Argument from Degree
The Argument from Degree states that some beings are more good, true, and noble, while some beings are less good, true, and noble. This is predicated by the way in which they resemble something that is maximally good, true, and noble. For example, a thing is said to be hot according to how it resembled something that is hottest, which is fire. So there must be something which is to all beings the cause of their being and every other perfection. This is called God.
5) The Teleological Argument
The Teleological Argument notes that things which lack intelligence act for an end. For example, the way the planets move around the sun. Things that lack intelligence cannot move towards an end unless directed by an intelligent being. Therefore, an intelligent being exists who directs all natural things. This is God.
What Does This Mean to Your Faith?
These arguments have by no means convinced every thinker throughout history, and many have tried to come up with refutations against them. In the end, these five proofs of God will likely impact you based on your own personal faith and spiritual experiences so far. If God is active in your own life, then they will likely serve as further proof of His existence.
Source
Thomas Aquinas, Saint. Summa Theologica. Trans. Father of the English Dominican Province. Benziger Bros, 1947.