Christianity isn’t founded in a philosophical perspective that evolved into a religion. Christian faith resulted from the revelation of God to the human race through Jesus Christ.
The Gospels make it clear that Jesus’ disciples misunderstood Him throughout His life. They thought that, as the promised Messiah, He would use supernatural power to set up an earthly kingdom. Consequently, when He was arrested and crucified, they lost hope (Matthew 26:56, 69-75). But at this point of despair and hopelessness, God revealed His redemptive plan. Jesus rose from death and physically appeared to His disciples in a glorious form (Luke 24:36-49; 1 Corinthians 15:1-8).
In the face of such a stupendous event, the disciples no longer had doubts regarding Jesus’ identity. Thomas, who was absent when Jesus first appeared, believed the testimony of His resurrection was too good to be true (John 20:24-26). But when he found himself face-to-face with Jesus, his response was simply: “My Lord and my God!” (John 20:28).
The apostles believed that Jesus is divine, along with the Father and the Holy Spirit (John 1:33-34; 14:16, 26; 16:13-15; 20:21-22). They believed in the divinity of the Father, Son, and Holy Spirit without ever questioning the foundational biblical truth that God is One (Exodus 20:2-3; Deuteronomy 6:4; Mark 12:29; 1 Corinthians 8:4, 6; Ephesians 4:3-6; James 2:19).
The starting-point of the Trinity is, naturally, not a speculative one, but the simple testimony of the New Testament. We are not concerned with the God of thought, but with the God who makes His Name known. But He makes His Name known as the Name of the Father; He makes this Name of the Father known through the Son; and He makes the Son known as the Son of the Father, and the Father as Father of the Son through the Holy Spirit. These three names constitute the actual content of the New Testament message. This is a fact which no one can deny (Emil Brunner, The Christian Doctrine of God, “Dogmatics,” vol. 1).
Although the biblical writers don’t use the terms Trinity or triune God, the Bible clearly teaches that God exists in three Persons: the Father, the Son, and the Holy Spirit (Matthew 3:16-17; 28:19; 2 Corinthians 13:14; Ephesians 2:18; 1 Peter 1:2). Each of these divine persons has His own personal characteristics and is clearly distinguished from the other persons (John 14:16-17, 26; 15:26; 16:7-15). Each divine person is equal in power, being, and glory, and each person is called God (John 6:27; Acts 5:3-4; Hebrews 1:8). Each has divine attributes (Hebrews 9:14; 13:8; James 1:17), and each performs divine works and receives divine honors (John 5:21-23; Romans 8:11; 2 Corinthians 13:14). In regard to His being or essence, God is one; but with respect to His personality, God is three.
This issue is basic to Christian faith. The doctrine of the Trinity (like the doctrine of the incarnation to which it is closely related) expresses some of the most profound and mysterious truths about God and His relationship to His creation. As the great church leader Athanasius pointed out, our salvation depends upon the incarnation. If Jesus were not both truly God and truly man, His death wouldn’t be sufficient to atone for our sin.
The Evangelical Dictionary of Theology, edited by Walter A. Elwell, gives the following concise definition of the Trinity:
Within the one essence of the Godhead we have to distinguish three “persons” who are neither three Gods on the one side, nor three parts or modes of God on the other, but co-equally and co-eternally God.
Although this theological definition is helpful, it is important to realize that none of us can have direct knowledge of God. His characteristics can only be described by analogy, and no analogy is perfect.