7. Revelation is the removal of a veil. When the discovery of truth is made by our natural powers, it is called natural revelation. By it man can easily know the existence of God as the First Cause and Master of all things, the Rewarder of good and evil the survival of the soul in another life of happiness or misery the principles of the moral law, in particular the duty of worshipping and serving God, etc. These truths have been known in all ages by all men who had the full use of reason. St. Paul, in his Letter to the Romans, speaking of the ungodly, writes "The invisible things of Him (of God), from the creation of the world, are clearly seen, being understood by the things that are made; His eternal power also, and Divinity: so that they are inexcusable. Because that, when they knew God, they have not glorified Him as God, nor given Him thanks" (I, 20, 21). And of the moral law he says that even the gentiles have the law written in their hearts, their conscience bearing witness to them (II, 15). Many other truths concerning God can he known by reason; as is proved in Natural Theology, a division of Metaphysics.
8. The word "revelation" is however more commonly used in another meaning and it is in this latter sense that we shall take it throughout this book; namely, to designate a manifestation of truth by God to man by a light superior to reason. In this meaning it is properly called "supernatural revelation". It is supernatural, because such light is not part of our mature, nor due to it, nor attainable by its unaided power. It supposes a special action of God announcing the truth to man. He has made this announcement through Prophets, Apostles, and other sacred writers, but especially through His Divine Son. He has thus taught us that we are destined to a supernatural happiness to which our nature cannot possibly give us a claim, and which is to consist in our seeing God face to face. A supernatural end cannot be reached but by supernatural means which our nature by its own powers can neither discover nor employ (II. 172).
9. To make known to us our supernatural end and the means of attaining thereto, a supernatural revelation was, therefore, absolutely necessary. Though it is not thus necessary for the knowledge of natural truths, even of such as regard religion and morality, still many difficulties impede the acquisition of such knowledge by unaided rcason. In particular, very few men have the talent and the opportunity to study such subjects deeply; and, even under the most favorable circumstances, owing to the depravity of the human hmeart, there always have been doubts and errors on many important points of morality and religion. This is abundantly proved by the history of past ages; and it is seen even to-day in the teachings of various philosophic systems which deny, or at least question, our most important duties to God. Therefore, a supernatural revelation is, not indeed absolutely, but yet relatively necessary for the proper understanding even of the natural law; it is necessary considering the condition of mankind. It may also be called morally necessary, the necessity arising from the fact that, while there is no physical impossibility, yet there is a very great difficulty in acquiring such knowledge as man needs to lead a life worthy of himself and of his Creator. Those who reject revelation are fond of calling themselves as if they were more rational than other men, while they are so irrational as to refuse additional light when it is offered them; and thus they act most rashly in matters in which the highest interests of man are concerned.
10. When we know a fact or a truth, whether by our natural powers or by revelation, we may still fail to see how the matter can be explained. It is then called a mystery: a natural mystery, if we arrive at the knowledge of its existence by our natural powers; a supernatural mystery, if by revelation only. That the scenes which we have formerly witnessed are recorded in our memory, we know; but how they are there recorded, is a natural mystery; how the three Divine Persons are one God, is a supernatural one. It is absurd for any one to deny that there are natural mysteries; a fortiori, we cannot deny that there are supernatural ones: for the things of God must necessarily be more incomprehensible to us than the sensible things around us. "The things that are of God," says St. Paul, "no man knoweth but the Spirit of God"; and he adds that we have received this Spirit, "that we may know the things that are given us from God" (1 Cor. II, 11, 12). We have then no right to refuse acceptance of a revelation, on the plea that it contains mysteries.