That there is one God is the continuous insistence, in the Old Testament as well as the New, that “The Lord our God is one Lord,” as Deuteronomy 6:4, the famous Shema Yisrael, reminds believers. The divinity of God the Father is equally clear in both Testaments (cf. Mal. 2:10, John 20:17).
Nevertheless, there are also hints, from the very beginning of the Old Testament, of a plurality of Persons within the Godhead. This can be seen as far back as the divine utterance in Genesis 1:26, “Let us make man in our image, after our likeness.” The Old Testament also contains hints that the coming Messiah would be divine (see, e.g., Isa. 9:6, Mal. 3:1).
Over the course of his public ministry, Jesus slowly unpacks the meaning of these passages by revealing his own divinity. As the apostle John would later write: “In the beginning was the Word, and the Word was with God, and the Word was God. He was in the beginning with God; all things were made through him, and without him was not anything made that was made” (John 1:1-2).
The apostles eventually grasp this, with St. Thomas being the first to proclaim him “my Lord and my God!”(John 20:28). From this point forward, Jesus’ divinity is part of the clear Christian message. We find the first Christian martyr, St. Stephen, praying, “Lord Jesus, receive my spirit,” and “Lord, do not hold this sin against them,” paralleling Jesus’ own prayers to the Father from the cross (Acts 7:59-60; Luke 23:34, 46).
Gradually, it becomes clear that, in addition to the Father and the Son, there is also a divine Holy Spirit. Since “the Spirit does not speak of himself” (CCC 687), the scriptural evidence of the Holy Spirit’s personhood and divinity is less direct than for the Father or the Son. The clearest evidence is probably St. Peter’s rebuke to Ananias: “Satan filled your heart to lie to the Holy Spirit. [. . .] You have not lied to men but to God” (Acts 5:3, 5).
Yet the Father, the Son, and the Holy Spirit are clearly distinct. Jesus’ promise at the Last Supper makes this clear: “I will pray the Father, and he will give you another Counselor to be with you for ever [. . .] the Counselor, the Holy Spirit, whom the Father will send in my name, he will teach you all things, and bring to your remembrance all that I have said to you” (John 14:16, 26).
So, there is one God. And yet the Father, the Son, and the Holy Spirit are each divine and not simply different names for the same Person.
How can all of this biblical data be harmonized? In the doctrine of the Trinity, which holds (in the words of the Athanasian Creed), that “the Father is God, the Son God, and the Holy Spirit God; and yet not three Gods but one God.” More specifically, the Godhead consists of three Persons (Father, Son, and Holy Spirit) united in a single substance.
What does this distinction mean? St. Basil the Great gives a helpful example: if we refer to “a man,” we’re pointing to his substance, what he is. If we refer to “Paul,” we’re referring to his person, who he is. So, there are three “Whos” in God but only one “What.” The Father is not the Son is not the Spirit, but all of them are equally God, equally uncreated, and equally glorious.
Because this is the synthesis of the entirety of God’s self-revelation, there’s no simple answer to the question, “Where does the Bible teach the Trinity?” The concept of the Trinity is revealed throughout Scripture, but the word Trinity is not. St. Thomas Aquinas explained that “the urgency of confuting heretics made it necessary to find new words to express the ancient faith about God.” After all, the theological debates are not over what Scripture says but what those sayings mean.
St. Jerome, scoffing at the Bible-quoting heretics of his day, said, “Let them not flatter themselves if they think they have [scriptural] authority for their assertions, since the devil himself quoted Scripture, and the essence of the Scriptures is not the letter, but the meaning.” The various attempts to assemble the pieces into any doctrine other than the Trinity have invariably led to heresy, failing to properly account for (or understand) some part of God’s revelation.
The doctrine of the Trinity is not simply an academic exercise. A proper Trinitarian understanding underscores any authentic personal relationship with Jesus Christ or the Holy Spirit. It is reflected in the theological claim “God is love” (1 John 4:8). From all eternity, God’s nature has been selfless love, because He is three Persons eternally loving one another. This is the heart of God’s self-revelation.