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