How Do We Know That God Exists?*
We know that God exists from our observation of the world around us and by the use of our rational faculties (reason) as human persons.
Observation
Everywhere in nature we find beauty and order. To try and explain this beauty and order as a result of chance is foolish. The incredible beauty of the world and its intricate order are obvious signs of an intelligence at work. The magnificence of a sunset coloring up an evening sky is at once beautiful to behold and yet fascinating to comprehend—the various gases of our atmosphere combined with heat, moisture and altitude all harmoniously working together for a few moments of glory. The awesome grandeur of the Grand Canyon Niagara Falls or the Rocky Mountains leaves us speechless, while the details of their coming into being boggles our minds. Because of our observation of beauty and order in nature, we conclude that only a living, intelligent being could have created the universe.
Comment: A missionary priest who spent many years in Africa recalls that he never had a problem convincing the natives that God existed. They believed in God long before the missionaries came to Africa. When he presented the Christian belief in God, they were converted because this Christian God fulfilled their vague notions of who God is. Missionary priests made their first converts into catechists who took a catechism in hand to far off villages to present the Catholic faith to the natives. Once they accepted the teachings in the catechism they were tested in their knowledge and acceptance of the truths of the Church. Then they were baptized by the missionary priests.
Reason
Only the human person among all God’s creatures has the ability to reason. This special attribute also allows us to know of God’s existence. Our desire for truth, our sense of moral righteousness, the call of our own consciences leads us to reason that there is more to our existence and to life in general than meets the eye. Through this openness we perceive signs of our spiritual souls.
St. Thomas Aquinas presented proofs from reason of the existence of God. See quotation below from:
I answer that, The existence of God can be proved in five ways.
The first and more manifest way is the argument from motion. 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. Thus that which is actually hot, as fire, makes wood, which is potentially hot, to be actually hot, and thereby moves and changes it. Now it is not possible that the same thing should be at once in actuality and potentiality in the same respect, but only in different respects. For what is actually hot cannot simultaneously be potentially hot; but it is simultaneously potentially cold. It is therefore impossible that in the same respect and in the same way a thing should be both mover and moved, i.e. that it should move itself. Therefore, whatever is in motion must be put in motion by another. If that by which it is put in motion be itself put in motion, then this also must needs be put in motion by another, and that by another again. But this cannot go on to infinity, because then there would be no first mover, and, consequently, no other mover; seeing that subsequent movers move only inasmuch as they are put in motion by the first mover; as the staff moves only because it is put in motion by the hand. Therefore it is necessary to arrive at a first mover, put in motion by no other; and this everyone understands to be God.
The second way is from the nature of the efficient cause. In the world of sense we find there is an order of efficient causes. 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 be 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. Now to take away the cause is to take away the effect. Therefore, if there be no first cause among efficient causes, there will be no ultimate, nor any intermediate cause. But if in efficient causes it is possible to go on to infinity, there will be no first efficient cause, neither will there be an ultimate effect, nor any intermediate efficient causes; all of which is plainly false. Therefore it is necessary to admit a first efficient cause, to which everyone gives the name of God.
The third way is taken from possibility and necessity, and runs thus. 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 time 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 to exist by something already existing. Therefore, if at one time nothing was in existence, it would have been impossible for anything to have begun to exist; and thus even now nothing would be in existence — which is absurd. Therefore, not all beings are merely possible, but there must exist something the existence of which is necessary. But every necessary thing either has its necessity caused by another, or not. Now it is impossible to go on to infinity in necessary things which have their necessity caused by another, as has been already proved in regard to efficient causes. Therefore we cannot but postulate the existence of some being having of itself its own necessity, and not receiving it from another, but rather causing in others their necessity. This all men speak of as God.
The fourth way is taken from the gradation to be found in things. Among beings there are some more and some less good, true, noble and the like. But "more" and "less" are predicated of different things, according as they resemble in their different ways something which is the maximum, as a thing is said to be hotter according as it more nearly resembles that which is hottest; so that there is something which is truest, something best, something noblest and, consequently, something which is uttermost being; for those things that are greatest in truth are greatest in being, as it is written in Metaph. ii. Now the maximum in any genus is the cause of all in that genus; as fire, which is the maximum heat, is the cause of all hot things. Therefore there must also be something which is to all beings the cause of their being, goodness, and every other perfection; and this we call God.
The fifth way is taken from the governance of the world. We see that things which lack intelligence, such as natural bodies, act for an end, and this is evident from their acting always, or nearly always, in the same way, so as to obtain the best result. Hence it is plain that not fortuitously, but designedly, do they achieve their end. Now whatever lacks intelligence cannot move towards an end, unless it be directed by some being endowed with knowledge and intelligence; as the arrow is shot to its mark by the archer. Therefore some intelligent being exists by whom all natural things are directed to their end; and this being we call God.
Thus through observation and reason we are able to discern that we are part of an existence far greater than ourselves. Humans throughout history have identified the existence of a source, a force, a being that knows no time, is not defined by our limits, and is both the cause and final goal of all life. In the Judeo-Christian tradition, we have consistently called this being God.
Comment: Observation and reason are supports to faith. Agnostics and atheists refuse to accept observation and/or rational arguments and, therefore are not convinced of (agnostics) or deny (atheists) the existence of God. The Catechism of the Catholic Church (#143) states that: “By faith, man completely submits his intellect and his will to God. With his whole being man gives his assent to God the revealer. Sacred Scripture calls this human response to God, the author of revelation, ‘the obedience of faith.’” (Cf. Rom 1:5; 16:26). In this way, each person must decide as an act of the will to believe or not to believe in God.
*Portions of text in italics are quoted from: Frs. Gerald Weber and James Killgallon: Life in Christ. New York, HarperSanFrancisco, 1995, pp. 9-10.