"Oh! ... The riches of the wisdom and the knowledge of God.
It is possible for people to know of God’s existence, even without the benefit of divine revelation—that is to say that we can know of God’s existence through our natural reasoning abilities, even if He had never told us anything about Himself. The inquisitive person can reason that God exists by examining His handiwork in the universe. The fact that there is movement, and causality, and order in the universe demands a Prime Mover, a First Cause, and a Designer and Governor, Whom we know by the name of God. We know that mankind is capable of this knowledge because we have seen it happen in history. Men like Plato and Aristotle, even though raised in a pagan culture, came to this natural knowledge of God. They did so, even without the revelation that we find in the Old Testament.
But, while man can know that there is a God, the existence of God in Trinity—the mystery we celebrate today—remains just that, a mystery. We know that God exists as Father, Son, and Holy Ghost, only because He has revealed this to us. No amount of deep thinking by the greatest of Greek philosophers could have ever figured this out through unaided human reason.
Even God’s revelation in the Old Testament was inadequate to the task—although we do hear a few clues; just hints, really, which, in retrospect suggest the Trinitarian existence of God. For example, in Genesis we read that “the spirit of God moved over the waters,” even before the creation of light. The Prophet Isaias predicted that “a Virgin would conceive, and bring forth a Son,” whose name would indicate that “God is with His people.” A little later on, the same Isaias predicted a descendant of King David upon whom would rest the spirit of the Lord, having powers which sound identical to the gifts of the Holy Ghost. But these were no more than hints, and no one could be expected to know of the existence of the trinity from them. In the Old Testament, we know of God only in His oneness.
But God was anxious for His children to know Him, and “in the fullness of time sent His only-begotten Son, so that we might know Him and believe in Him; and in believing Him we might not perish, but have life everlasting.” And part of that “knowing and believing” is in knowing and believing that He exists in the Trinity of Father, Son, and Holy Ghost.
The revelation of God’s Son was quite direct. The Son of God was conceived in the womb of the Blessed Virgin, predicted, as we said, by the Prophet Isaias—and He was born in Bethlehem, as was predicted by another prophet, named Micheas. When He reached maturity He led a public life, preaching and curing the sick; and was ultimately crucified and resurrected from the dead. We can read about this second person of the Blessed Trinity in the Gospels and the other Books of the New Testament.
The revelation of the Third Person, the Holy Ghost, was a little less direct. Toward the end of His time on earth, the Son told His followers that He would be returning to His Father in heaven, but that He would send someone in His place, to be their Advocate, the Spirit of Truth, the Holy Ghost. And just last Sunday we saw how that third Divine Person came upon the Apostles and the Mother of God some fifty days after the Son’s resurrection. And we know, as well, that we receive that same Holy Ghost in the Sacrament of Confirmation, and that He dwells within us when we are in the state of grace.
We see in today’s Gospel that it was the command of the Son of God Himself that this revelation of God and the revelation of His Commandments be brought to all nations with the hope that they will be baptized “in the name f the Father, and of the Son, and of the Holy Ghost.” The obligation is a serious one, for we read, elsewhere, our Lords words: “He who believes and is baptized shall be saved, but he who does not believe shall be condemned.
The collect this morning speaks to the fact that God has “given His people the true Faith, to adore the glory of the eternal Trinity, and to adore the unity of the power of His majesty.” That is an important concept, often forgotten in the modern world—the concept there is a “true Faith.” Actually, there should be no surprise in this, except that our thinking is often blurred by the modernism of the age. Our “true Faith,” is not something that we have decided for ourselves. Not something organized by a committee. Not the latest consensus of what people think at the moment. It is not even limited to the wisest speculation of the brightest philosophers. It has been revealed to us by God Himself, who can neither deceive nor be deceived; It has been revealed to us by Truth Himself, and confirmed by the Spirit of Truth. With their help, the true Faith has “come down to us from the Father of Lights, with whom there is no change, nor shadow of alteration.” Thus the unique and unchanging nature of the true Faith.
This feast day of the Holy Trinity, then, has two important characteristics, which we ought to take to heart and act upon: the first characteristic is intimacy with God; the second is a missionary imperative.
We can speak of intimacy with God, because this revelation of the Trinity, which we could never know through natural reason, is very much like being invited into God’s household, and getting to know the family of that household as a friend and invited guest—or, more correctly—as an adopted son or daughter; a member of God’s family! And, of course, that is a privileged relationship that we will want to protect and even to cultivate, by keeping God’s Commandments, and by getting to know Him better in prayer and in the Sacraments.
And, finally, there is also a missionary dimension in this true Faith. God has directed us to bring it to all nations—to all of the peoples of the earth. For some Catholics that will mean actual excursions across the seas to foreign lands where they will preach in foreign languages to people of different cultures. For others, the mission field will be right here where we live—for there will always be a need for the true Faith to be preached right at home; priests, religious, and lay catechists.
The vast majority of Catholics will not be called upon to do any preaching or missionary work in the formal sense. But that doesn’t mean that anyone is without obligation. All of us are always obligated to spread the true Faith through our good example—things like the cheerful keeping of the Commandments, personal honesty and integrity, and an enthusiasm for holy things. An important part of that good example must come from the recognition that we have the true Faith. Our Faith is not just the work of wise and prudent men—no, it is the revelation of God Himself about Himself. “Those who believe and are baptized will be saved.” To those who believe the words of Truth Himself, He has promised that He would “be with us all days, even to the consummation of the world.”
Keep the Faith! The one true Faith, by which we acknowledge the glory of God, the Faith by which God will protect His children from adversity.
 Epistle: Romans xi: 33-36.
 Cf. Saint Thomas Aquinas, Summa Theologica, Ia, Question 2.
 Genesis i: 2.
 Isaias vii: 14.
 Isaias xi: 1-3.
 John iii: 16-17.
 Micheas v: 2.
 John xiv, xv.
 Acts ii.
 Cf. Catechism of the Council of Trent, Chapter on Confirmation. http://www.cin.org/users/james/ebooks/master/trent/tsacr-c.htm.
 Gospel, Matthew xxviii: 18-20.
 Mark xvi: 14-20 (Gospel of Ascension Thursday).
 Collect, Trinity Sunday.
 James i: 17,