“I don’t believe in the Trinity,” my friend said. She and I were discussing the Christian doctrine that holds that one God subsists in three persons: Father, Son, and Holy Spirit. Despite being a Christian believer, she rejects the doctrine of the Trinity. I was surprised and concerned that she rejected what I believed to be a cornerstone doctrine.

Wondering about her Trinitarian unbelief, I determined to delve into this “problem which has long vexed the Church, and which even now has not been solved to the satisfaction of all who bear the Christian name,” according to Yale historian Kenneth Latourette.

“Doctrines are not God: they are only a kind of map,” C. S. Lewis observed. He compared God to the Atlantic Ocean and theology to a map of the Atlantic Ocean. The map is not the Atlantic Ocean any more than theology is God, but the map is necessary if we want to go anywhere. Lewis argued that theology is practical, yet “bound to be difficult, at least as difficult as modern Physics.” We can expect theology to be difficult and complex, yet necessary if we hope to go anywhere with our faith.

As my friend rightly noted, the Bible nowhere contains the word “Trinity.” An easy response, though, is that many bedrock Christian doctrines are given names that are not found in the Bible, such as “monotheism,” “incarnation,” or “divinity.” For that matter, the entire Book of Esther does not contain the word “God.”

But my friend’s objection hinted at a deeper question. How did the church discern the doctrine of the Trinity?

To begin with, the church had the fact of the historical Jesus, and the fact of the one God who spoke through the Hebrew scripture. The New Testament bore witness to the Holy Spirit.

Latourette describes how early Christian theologians faced the question of how to put the fact of Christ “into the categories of existing human knowledge, thought and speech.” Tertullian (c. 160 – c. 220 A.D.), the father of Latin theology, is credited with inventing the term trinitas. To translate the Greek term hypostasis he introduced the term persona, which translates into English as “person.” The term persona means a mask, and it calls to mind the masks that actors in Roman dramas wore to distinguish the different characters they portrayed. This gives the image of one actor who plays different roles.

Tertullian also introduced the term substantia to mean the one substance that the Father, Son, and Spirit share.

At the First Ecumenical Council called by Constantine at Nicea in A. D. 325, the bishops agreed that the Son is consubstantial with the Father. This doctrine countered the Arian heresy that Christ was not divine. The Bible tells us that Jesus is the only begotten son of God. “To beget is to become the father of: to create is to make,” Lewis explained. “When you beget, you beget something of the same kind as yourself.”

The Second Ecumenical Council at Constantinople in A. D. 381, resolved that Christ is “the only-begotten Son of God, eternally begotten of the Father, light from light, true God from true God, begotten not made, consubstantial with the Father.” The Second Ecumenical Council also concluded that “the Holy Spirit, the third person of the Trinity, is God, one and equal with the Father and the Son, of the same substance and also of the same nature.”

Theologian Alister McGrath recounts that the bishops and early church fathers reached this understanding of the doctrine of the Trinity by looking at the scripture, including Matthew 28:19 (“baptizing them in the name of the Father, of the Son, and of the Holy Spirit”) and 2 Corinthians 13:14 (“May the grace of the Lord Jesus Christ, and the love of God, and the fellowship of the Holy Spirit be with you all”), for signs of God’s “pattern of divine activity.”

Today, Christians everywhere view the Trinity as foundational. Catholics believe that “the faith of all Christians rests on the Trinity,” which represents “the central mystery of Christian faith and life.” Similarly, Baptists hold that “the eternal triune God reveals Himself to us as Father, Son, and Holy Spirit, with distinct personal attributes, but without division of nature, essence, or being.”

My friend’s second objection concerned the conflict between the Son’s apparent subordination to the Father in the biblical narrative versus the Trinitarian doctrine of equality among Father, Son, and Holy Spirit. Augustine took up this objection: “Although the Son and the Spirit may appear to be posterior to the Father, this judgment only applies to their role within the process of salvation. Although the Son and Spirit may appear to be subordinate to the Father in history, in eternity all are co-equal.” Viewed in this light, any appearance of inferiority of the Son and Spirit to the Father merely reflects our limited human condition and does not take into account the eternal nature of God.

Her final objection is probably the most difficult to answer. How do you reconcile Old Testament monotheism with a triune God?

We must frequently hold two principles in tension. Two opposing heresies lie on either side of orthodox Trinitarian doctrine. The first heresy, Modalism, claims that there are three terms for the same God, and the only difference is where this God appears and at what time. The second heresy, Tritheism, asserts that there are three equal, independent and self-sufficient beings who are all divine. Both of these heresies are quite a bit simpler and easier to grasp than the Trinity, but each one lacks an essential element (three persons in the case of Modalism; one God in the case of Tritheism).

Lewis used the illustration of a straight line in one dimension. In two dimensions, the line can become a square. In three dimensions, the line can become a cube. Our human dimension is like the first dimension compared to God’s greater dimension. “On the human level, one person is one being, and any two persons are separate beings. In God’s dimension, so to speak, you find a being who is three Persons while remaining one Being, just as a cube is six squares while remaining one cube.” From outside God’s dimension, the mental experience resembles watching a 3-D movie without the glasses.

We cannot, in our human capacity, understand the Trinity. But we should contemplate the doctrine, and as Lewis wrote, “the thing that matters is being actually drawn into that three-personal life, and that may begin any time — tonight if you like.” As Lewis observed, “If Christianity were something we were making up, of course we could make it easier. But it is not. We cannot compete, in simplicity, with people who are inventing religions. How could we? We are dealing with Fact. Of course anyone can be simple if he has no facts to bother about.”

Or as my three-year-old daughter once cheerfully declared her understanding of the Trinity, “It doesn’t make any sense. But it’s true.”