The most obvious question anyone asks a believer is “How do you know God exists?” It’s a simple question that has both a simple and a complicated answer.
First, the easy part: faith is believing in something or someone you cannot see or believing in what cannot be proven. In other words, faith depends on not having any evidence; otherwise, it would not be faith. So those who believe in God or in a supreme being are taking the word of others or just trusting their own instincts that such an almighty divinity does exist. Many people, whether they are Jewish, Christian, or Islamic, whether Roman Catholic, Eastern Orthodox, or Protestant, believe in God because they believe in the revealed Word of God, called Sacred Scripture or the Bible.
A more complicated answer is that Catholic Christianity does believe you can prove the existence of God. Reason can conclude that a supreme being exists and is necessary, but only faith (believing what cannot or is not known) tells us that there is but one God (monotheism).
Judaism, Christianity, and Islam believe in only one God, and Christianity is the only of the three to believe that there are three Divine Persons in that one God. Human reason could never figure out the mystery of how there can be three persons but not three gods. Faith is needed to believe that doctrine, and some people never embrace that faith.
The fact that God exists can be known by reason alone, or it can be believed by faith. The ancient Greeks and Romans used philosophy (logic and reason) centuries before Christ to prove the existence of God or a supreme being. Saint Thomas Aquinas was a Catholic philosopher and theologian in the thirteenth century AD who used that same ancient reasoning to demonstrate the reasonableness of anyone “knowing” that God exists, regardless of whether or not they have faith. He showed that the existence of God can be proved by reason, but after that, one needed supernatural faith to believe the supernatural revelation about the nature of God (for example, the Trinity).
Aquinas used five proofs to demonstrate that, using human reason alone, any human being could know that there is a God. The first method is called motion. This is not physical movement from point A to point B on a map or on a road. It means moving from being purely potential to being actual. Philosophers call this motion “potency into act.”
So, for example, the sculpture of the Pieta done by Michelangelo was potential until it moved into actuality once the artist began chiseling the stone. The finished product was first potential and then made actual, but not by itself. The artist was the “mover,” that is, the one who moved the “potential Pieta” to become the “actual Pieta.” He did this by hammering away at the marble until he finished. Were there no artist, then the potential sculpture would remain potential forever. Only something already actual can move something else from being potential to being actual. You and I were potential human beings until our parents moved us into being actual once we were conceived.
Aquinas reasoned that the entire universe was potential (before the Big Bang) until something or someone moved it from potency to act (from being a potential universe to being an actual universe). Since everything has a beginning, then that must mean everything was potential at one time.
Only a “prime mover” or something always actual and never potential, could be considered a supreme being. If God had a beginning, then there would have been a time when there was no God; if that were the case, then who or what would have moved Him from being a potential God to being an actual God? Aquinas said reason compels us to discover there must have always been something which always existed, the Prime Mover, who moves everything from being possible to being real.
If your head does not hurt yet, hold on. The second proof is called causality. Every crime sleuth from Sherlock Holmes to Lord Peter Whimsey to Miss Marple solves the murder mystery based on the principle of cause and effect. A dead body is found in the room with five bullet holes and a knife in the back. That is the effect. Something and someone (the murderer) caused a living person to become a corpse. Bullets and knives do not kill people; people kill people.
Causality is the underpinning of science, logic, and most of our experiential knowledge. We see a burnt piece of paper (the effect) and automatically reason that something or someone was the source (the cause) of the burning. It is nothing more than connecting the dots, so to speak. Aquinas reasoned, therefore, that every effect has a cause; otherwise, it would not exist. You and I are effects, and our parents are the causes. He also reasoned that every cause was in turn an effect of a previous cause. Grandma and Grandpa had something to do with us, too, since without them (cause) they would have had no children (effect) and without those children growing up to become our parents (cause), there would have been no us (effect).
Therefore, if we reason backwards, like we did with motion, and deduce that every cause must have had a previous cause, there must be a first cause like there was the prime mover. This first cause is the cause of all causes. It had no cause since it has always existed. The Uncaused Cause, the Cause of All Causes and the Prime Mover, can be called God or the Supreme Being, if you like. Aquinas never claimed this was as theatrical as the burning bush that Moses encountered, but it makes sense.
The third proof is called necessity. As important as your boss thinks he is, in reality, the universe would not cease to exist if he ceased to exist—he is not necessary. But don’t tell him that unless you don’t want that raise you’ve been asking for! No matter how important, nothing in the world or in the universe is absolutely necessary. Everything is contingent; that means it does not have to exist in order for reality to exist. Only one being is necessary, and that is the Source of being itself, the Being that keeps everything in existence. Causality and motion merely explain how something got here, namely, everything is created and has an origin.
Necessity and contingency, on the other hand, show us that something or someone is needed to keep things going even after they are made. Edison may have invented the light bulb, but he died, and we still use electric lights well after his death. Electricity is needed, however, to keep any and all light bulbs working. Think of the Necessary Being, the Being itself, or the Existence itself (you can even call it God or the Supreme Being), as the electricity which keeps everything in existence. That which keeps in existence everything there is, is a necessary being. Everything which depends on that necessary being is contingent. Head hurt yet?
The fourth proof is called gradation. There is a hierarchy of being in the universe. First, at the most fundamental level, even at the subatomic, there is inanimate matter and energy. It exists, but it is not alive. Then you move up into the food chain and we have the second level of being: plant life (vegetative). Basic, simple, uncomplicated, plant life carries on three activities: nutrition, growth, and reproduction. The third level of being, however, is more complicated. While it has the same three principles, it also has sensation. Animal life is higher than plant life because these creatures have bodies which transmit information (data) from their senses to their brains. Though these brains act on instinct, when a sound is heard through their ears or images are seen by their eyes, animals react as instinct dictates.
This fourth level of being incorporates the first two but goes into a much more sophisticated mechanism. Human life not only consists of nutrition, growth, and reproduction (as does plant life), and has sense knowledge from the physical body (as does animal life), but it also involves the ability to reason. Human beings have rational intellects and free will. While animals act out of instinct, only men and women can make free choices and base them on reason. When was the last time you saw a hungry dog or cat turn away food just because they were on a diet and needed to lose weight? Pooch or Kitty will eat out of instinct whenever they are hungry. Human beings can resist hunger and freely choose to diet. The beings above humans are angels. These are beings which, unlike us, are not body and soul. They are pure spirit. They have intellect and will, like we do, but they are not limited by physical bodies which can feel hunger, pain, cold, heat, sickness, and death.
Angels cannot die since they are spiritual beings with no mortal bodies to hinder them. The last and highest level of being, however, is not angelic but divine. Angels may not die, but they were created. They are limited and finite, while the fullness of being is God. He is the highest, the Supreme Being. God has no beginning and no end. God always existed. Since He is everywhere (omnipresent) and knows everything (omniscient) and can do anything (omnipotent), He is the pinnacle and zenith of being itself.
The fifth and last proof is called governance. This has nothing to do with city hall, Parliament, Congress, or the White House. Something or someone with intelligence created a world and a universe which operates on intelligent and rational laws and principles. Whether you believe in Creationism or subscribe to a form of evolution, you recognize that there is a systematic plan upon which reality operates. The laws of physics and chemistry were not created by scientists; they were discovered by them. Those laws apply equally in any country and on any planet. Governance merely means an intelligent Being created the intelligent laws and systems that maintain order and prevent chaos. That Governor can be called God.
Need an aspirin now? No one ever claimed these five proofs would convert an atheist overnight. They do show, however, that the existence of God can be known by reason. The pagan Romans and ancient Greeks, for example, among many other peoples, figured out there was a supreme being even without having that deity reveal himself. Yet that is the limit of human reason. We can only know that there is a God. To know God and not just know about Him involves faith.
Faith allows you to believe in the truths revealed by God. Reason helps make some sense of that, but much of the mystery of religion does not contradict reason; rather, it goes beyond the limitations of human reason. Then you just have to believe.