"Unity" means to be one, and "Trinity," three in one.

21 Q. Is there but one God?
A. Yes; there is but one God.

22 Q. Why can there be but one God?
A. There can be but one God because God, being supreme and infinite, cannot have an equal.

"Supreme," that is, the highest. "Equal," when two are equal one has everything the other has. You could say one pen is the equal of another if it is just as nice and will write just as well; one mechanic is the equal of another if he can do the work equally well. Two boys are equal in class if they have exactly the same marks at the end of the month or year. You could not have two persons chief. For example, you could not have two chief generals in an army; two presidents in the nation, or two governors in a state, or two mayors in a city, or two principals in a school, unless they divide equally their power, and then they will be equals and neither of them chief. God cannot divide His power with anyone -- so as to give it away entirely -- because we say He is infinite, and that means to have all. Others have only the loan of their power from God. Therefore, all power and authority come from God; so that when we disobey our parents or superiors who are placed over us, we disobey God Himself.

23 Q. How many persons are there in God?
A. In God there are three divine persons really distinct and equal in all things -- the Father, the Son, and the Holy Ghost.

"Distinct," not mingled together. We call the first and second persons Father and Son, because the second is begotten by the first person, and not to indicate that there is any difference in their age. We always see in the world that a father is older than his son, so we get the idea perhaps that it is the same in the Holy Trinity. But it is not so. God the Father, and God the Son, and God the Holy Ghost existed from all eternity, and one did not exist before the other. God the Son is just as old as God the Father, and this is another great mystery. Even in nature we see that two things may begin to exist at the same time, and yet one be the cause of the other. You know that fire is the cause of heat; and yet the heat and the fire begin at the same time. Though we cannot understand this mystery of the Father and Son, we must believe it on the authority of God, who teaches it. First, second, and third person in the Blessed Trinity does not mean, therefore, that one person was before the other, or brought into existence by the other.

24 Q. Is the Father God?
A. The Father is God and the first Person of the Blessed Trinity.

25 Q. Is the Son God?
A. The Son is God and the second Person of the Blessed Trinity.

26 Q. Is the Holy Ghost God?
A. The Holy Ghost is God and the third Person of the Blessed Trinity.

27 Q. What do you mean by the Blessed Trinity?
A. By the Blessed Trinity I mean one God in three Divine Persons.

*28 Q. Are the three Divine Persons equal in all things? A. The three Divine Persons are equal in all things.

29 Q. Are the three Divine Persons one and the same God? A. The three Divine Persons are one and the same God, having one and the same divine nature and substance.

Though they are one and the same, we sometimes attribute different works to them. For example, works of creation we attribute to God the Father; works of mercy to God the Son; and works of love and sanctification to the Holy Ghost; and you will often find them thus spoken of in pious books; but all such works are done by all the Persons of the Trinity; because such works are the works of God, and there is but one God.

*30 Q. Can we fully understand how the three Divine Persons are one and the same God?
A. We cannot fully understand how the three Divine Persons are one and the same God, because this is a mystery.

"Fully" -- entirely. We can partly understand it. We know what one God is and we know what three persons are; but how these two things go together is the part we do not understand -- the mystery.

*31 Q. What is a mystery?
A. A mystery is a truth which we cannot fully understand.

"A truth," that is, a revealed truth -- one made known to us by God or His Church. It is a truth which we must believe though we cannot understand it. Let us take an example. When a boy goes to school he is taught that the earth is round like an orange and revolving in two ways, one causing day and night and the other producing the seasons: spring, summer, autumn, winter. The boy goes out into the country where he sees miles of level land and mountains thousands of feet in height. Again he goes out on the ocean where sailors tell him it is several miles in depth.

Now he may say: how can the earth be round if deep valleys, high mountains, and level plains prove to my senses the very opposite, and the countless things at rest upon its surface tell me it is motionless. Yet he believes even against the testimony of his senses that the earth is round and moving, because his teacher could have no motive in deceiving him; knows better than he, having learned more, and besides has been taught by others who after long years of careful study and research have discovered these things and know them to be true. If therefore we have to believe things that we do not understand on the authority of men, why should we not believe other truths on the authority of God? Yes, we must believe Him. If a boy knew all his teacher knew there would be no need of his going to school; he would be the equal in knowledge of his teacher, and if we knew all that God knows we would be as great as He. As well might we try to empty the whole ocean into the tiny holes that children dig in the sand by its shore, as fully to comprehend the wisdom of God. This is the mistake unbelievers make when they wish to understand with their limited intelligence the boundless knowledge and mysterious ways of God, and when they cannot understand refuse to believe. Are they not extremely foolish? Would you not ridicule the boy who refuses to believe that the earth is round and moving because he cannot understand it? As he grows older and learns more he will comprehend it better; so we, when we leave this world and come into the presence of God, shall see clearly many things that are unintelligible now. For the present, we have only to believe them on the authority of God teaching us. Another example. We take two little black seeds that look just alike and place them in the same kind of soil; we put the same kind of water upon them; they have the same sunlight and air, and yet when they grow up one has a red flower and one a blue. Where did the red and where did the blue come from? From the black seed, or the brown soil, or the pure water, air and sunlight? We do not know. It is there, and that is all. We see it and believe it, though we do not understand it.

So if we refuse to believe everything we do not understand, we shall soon believe very little and make ourselves ridiculous.