The Feast of the Holy Trinity follows Pentecost because it is only by the inspiration of the Third Person of the Trinity, who leads into all truth, that the mystery of the Trinity can be known. Human intelligence needs God’s help to apprehend the inner reality of God. Certainly, human reason can employ natural analysis to some extent to describe God in terms of causality and motion and goodness. Saint Anselm, who models the universality of Christendom by being both an Italian and an Archbishop of Canterbury, said that “God is that, than which nothing greater can be conceived.”
A house is a house because it houses. But what is in the house is known only by entering it. Since creatures cannot enter the Creator, he makes himself known by coming into his creation. “No one has seen God at any time. The only begotten Son, who is in the bosom of the Father, he has declared him” (John 1:18).
Had we invented the Trinitarian formula, it would be only a notion instead of a fact. There are just three choices: to acknowledge what God himself has declared, to deny it completely, or to change it to what makes sense without God’s help. That is why most heresies are rooted in mistakes about the Three in One and One in Three.
Unitarianism, for example, is based on a Socinian heresy. Mormonism is an exotic version of the Arian heresy. Islam has its roots in the Nestorian heresy. All three reject the Incarnation and the Trinity but selectively adopt other elements of Christianity. Like Hilaire Belloc in modern times, Dante portrayed Mohammed not as a founder of a religion but simply as a hugely persuasive heretic, albeit persuading most of the time with a sword rather than dialectic. These religions, however, are not categorically Christian heresies since “Heresy is the obstinate post-baptismal denial of some truth which must be believed with divine and catholic faith . . .” (Catechism, 2089). Only someone who has been baptized can be an actual heretic.
Cultures are shaped by cult: that is, the way people live depends on what they worship or refuse to worship. A culture that is hostile to the Holy Trinity spins out of control. In 1919, William Butler Yeats looked on the mess of his world after the Great War:
Things fall apart; the centre cannot hold;
Mere anarchy is loosed upon the world . . .
That is the chaotic decay of human creatures ignorant of their Triune God. “The best lack all conviction, while the worst / Are full of passionate intensity.” But to worship the “Holy, Holy, Holy” God as the center and source of reality is to confound anarchy: “For in Him all things were created, things in heaven and on earth, visible and invisible . . . He is before all things, and in him all things hold together” (Colossians 1:16-17).