*This is not meant to be an exhaustive demonstration, but merely an introduction…
Preambula Fidei, literally, the ‘walk before faith’ will introduce some preliminary truths which can be known by the light of reason alone without the aid of Divine Revelation.
The first principle that must be established is that truth does indeed exist, and secondly, that we are able to know the truth. The statement that “all truth is relative” is self-refuting. The statement itself would be ‘absolute’ and so would violate the meaning of the statement. If we can speak with any meaning at all, then truth–absolute truth–must exist. (Again, this is not meant to be an exhaustive demonstration).
What is the highest truth knowable by human reason alone? That God exists. God existence can be known in many different ways without the aide of Divine Revelation. The First Vatican Council teaches that:
The same Holy mother Church holds and teaches that God, the source and end of all things, can be known with certainty from the consideration of created things, by the natural power of human reason : ever since the creation of the world, his invisible nature has been clearly perceived in the things that have been made. [Romans 1:20]
Thus, is the highest form of philosophy (the ‘love of wisdom’) is metaphysics (the study of being). The Church has often referred to philosophy as the ancilla theologiae (“handmaid of theology”) because theology needs and uses reason. As the late Pope John Paul II explains in his encyclical Fides et Ratio (On the Relationship Between Faith and Reason):
Faith and reason are like two wings on which the human spirit rises to the contemplation of truth; and God has placed in the human heart a desire to know the truth—in a word, to know himself—so that, by knowing and loving God, men and women may also come to the fullness of truth about themselves (cf. Ex 33:18; Ps 27:8-9; 63:2-3; Jn 14:8; 1 Jn 3:2).
He also reminds us that theology needs philosophy to confirm the intelligibility and universal truth of its claims (cf. Fides et Ratio #77). Theology is not anti-reason, the common insult that Christians (or really, any person of faith) ‘checks [their brain] at the door’ is completely false. A theology that conflicts and contradicts reason is bad theology. Faith is not opposed to reason. We exercise faith every day of our lives, even the most militant atheist is a man of a faith though it is not exercised as religious faith. We believe things to be true every second of every day whether we freely admit it or not. When you walk into a room and flip the light switch, you believe the light will turn on (or off if you are in the habit of leaving your lights on). And you probably have really excellent evidence for believing the light switch will turn on the lights in the room… the light fixture is wired to the switch, and there’s a bulb in the light fixture. However, without having studiously examined both the fixture, the switch, and all the little pieces and parts that connect to make the switch work, you do not know upon walking into the room and flipping the switch that it will work… you believe (and have a kind of faith) that things are working correctly and that when you flip the switch the light will indeed turn on. The same principle (of a kind of basic faith) holds true while you are working and your car is in the parking lot… you cannot see your car (and someone may have stolen it), yet you believe your car in there in the parking lot and if asked you would say that your car is parked in the lot, even though you cannot at that moment in time see your car parked there. You believe (on good evidence) that your car is there and will still be there until you drive it away. Thus, faith is not in and of itself unreasonable.
Keeping that in mind however, as stated above, we can also say that God’s existence can be known without theology. That is, God’s existence is not solely an exercise of faith. St. Thomas Aquinas, St. Anselm and several others have provided clear demonstrable arguments for God’s existence based purely on reason.
St. Thomas Aquinas outlines his infamous Five Ways in his Summa Theologica. My (personal) favorite of the Five Ways is the third way…
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.
Basically, there are things in the world that come to be and then pass out of existence and so are able to not exist (i.e., they are contingent beings). Because these contingent beings can not exist, they must receive their existence from another and not from themselves. This cannot continue to infinity (because an infinite regress explains nothing). Thus, there must be some being whose existence is of itself and not given to it– it possesses existence by its very nature and so is a necessary being. This being cannot not exist. And this is what we call God.
Yes, it is a big leap to jump from the discovery of God–that God exists– to the claims of religious faith about God. Namely, that God is a personal God, that God is intimately involved in our very lives so much that while we were still sinners Christ died for us (cf. Romans 5:8). But I think that upon examination of the historical evidence regarding the person of Jesus, it should not be considered ‘unreasonable’ to believe in Him. Yes, belief in the person (and claims) of Jesus Christ is an act of complete faith… but it is not without evidence.
Summa Theologica – St. Thomas Aquinas
Proslogion – St. Anselm
God is No Delusion – Fr. Thomas Crean, OP
The Order of Things – Fr. James V. Schall