In one sense, the Trinity is easy to explain: Christians believe that there is only one God, but they also believe that the Father is God, the Son is God, and the Holy Spirit is God (Acts 5:1-4). God exists as three persons, but he is not three gods. He is instead one God who exists as three equally divine, equally eternal persons, and this truth forms the central foundation and mystery of the Christian faith (Catechism of the Catholic Church 234).
But in another sense, it’s not easy to explain how God can be one infinite being and three persons at the same time. Analogies that Christians often employ can easily lead to heresies if they’re taken too literally. These include the three-leaf clover, which can lead to God existing as three separate parts (or tri-theism), and water’s ability to be solid, liquid, and gaseous, which can lead to turning the Trinity into three modes of God’s existence instead of three persons (or modalism).
At some point, the theologian must admit that we can’t fully explain how God exists as a Trinity and that this is a mystery of the Faith, but this isn’t a cop-out in order to avoid answering important questions and objections.
Many religious critics of the Trinity, such as Muslims, Jews, Mormons, and Jehovah’s Witnesses, also believe in divine attributes that generate puzzles, but they don’t abandon those doctrines just because human reason can’t completely comprehend them. These “Unitarians” shouldn’t scoff at the mysterious nature of the Trinity—after all, I highly doubt they could explain how God can make something from nothing and immaterially exist in all places without having to admit that these divine abilities are mysterious in some way.
Other criticisms of the Trinity confuse something being mysterious with its being nonsensical and thus unworthy of belief. For example, one Watchtower article says, “The Trinity, explain Catholic scholars Karl Rahner and Herbert Vorgrimler, ‘could not be known without revelation, and even after revelation cannot become wholly intelligible.’ Can you really love someone who is impossible to know or understand? The doctrine of the Trinity, therefore, is a barrier to knowing and loving God.”
But being incomprehensible does not entail being unintelligible.
The Trinity cannot be fully comprehended, or understood in every respect. But just because something is not “completely intelligible,” it does not follow that it is unintelligible or nonsense. Jehovah’s Witnesses even admit that their God Jehovah is not completely understandable. Reasoning from the Scriptures says, “Should we really expect to understand everything about a Person who is so great that he could bring into existence the universe, with all its intricate design and stupendous size?”
The Trinity is a mystery, but that does not mean that it is some unknowable “black hole.” Rather, a theological mystery refers to truths we would not know if God had not revealed them to us (CCC 237). It is, like other mysteries of the faith, “not taught by human wisdom but taught by the Spirit” (1 Cor. 2:13). While this doctrine was revealed implicitly in the Old Testament, it becomes explicit in the New Testament through Christ and the apostles’ teachings about the oneness of God (John 17:3; 1 Tim. 1:17), the deity of Christ (John 1:1; Titus 2:13), and the deity of the Holy Spirit (Acts 5:1-4).
The Old Testament already deepened our understanding of God as father—not as a literal man (Num. 23:19), but as the creator of the universe through whom all things owe their existence. But even apart from creation, God is eternally Father in relation to his only-begotten, eternal Son, who is equally divine. And he is the Father who sends the Holy Spirit through the Son so that his work of creation, and the Son’s work of redemption, is made complete in the Spirit’s work of sanctification, or in making us holy (CCC 243).
Finally, the doctrine of the Trinity helps remove some of God’s mystery by explaining how God can be not just perfectly loving, but love itself (1 John 4:18).
I once watched a Catholic from Lebanon engage a Muslim friend on the subject of whether Allah could be three persons. He asked his friend, “Does Allah perfectly love?” to which his friend said, “Of course!” My Catholic friend then posed this question, “Well, then, whom did he love before he created the world?”
This Muslim couldn’t answer the question. He could have said Allah (the Muslim word for God) perfectly loves himself. But most of us understand that love in its truest and highest form involves the giving of self to another person. In that sense, theologians have said the Trinity represents the Father’s and Son’s perfect love for one another. The Father eternally begets the Son, so there was never a time when the Father was without the Son (in Trinitarian theology, the Son’s relationship with the Father is called generation).
Moreover, the Father and the Son look upon one another from all eternity, beholding each other in perfect love, and utter what we might call a “sigh of gladness.” The Roman Catechism says the Holy Spirit “proceeds from the divine will inflamed, as it were, with love,” and from this sigh, the Holy Spirit has existed for all eternity in relationship between the Father and the Son. (In trinitarian theology, the Holy Spirit’s relationship with the Father and the Son is called spiration, from the Latin aspire, which means “to breathe.”)
“O blessed light, O Trinity and first Unity!” God is eternal blessedness, undying life, unfading light. God is love—Father, Son, and Holy Spirit. God freely wills to communicate the glory of his blessed life. Such is the “plan of his loving kindness,” conceived by the Father before the foundation of the world, in his beloved Son: “He destined us in love to be his sons” and “to be conformed to the image of his Son,” through “the spirit of sonship” (CCC 257).