The belief that God exists as Trinity distinguishes Christianity from other religions.
You might be surprised that the word Trinity appears nowhere in the Bible. The concept of the Trinity, however, fills the pages of Scripture. The Old Testament hints at God’s Triune nature. We see “the Spirit of God” hovering over the primordial waters of creation (Genesis 1:2). God at times refers to himself using a plural pronoun (e.g. Genesis 1:26). The Angel of the Lord in the Old Testament is sometimes identified as God, while other times he is distinguished from God (Genesis 16:7-14).
In the New Testament, we have a fuller revelation of God. The Apostle John begins his Gospel, saying, “In the beginning was the Word, and the Word was with God, and the Word was God (John 1:1).” Jesus of Nazareth claimed to be God (John 14:9). Paul said of Jesus, “The Son is the image of the invisible God, the firstborn over all creation (Colossians 1:15).” The Holy Spirit takes the place of the Son, when the Son ascended to heaven, to bring completion to the Son’s work on earth (John 16:8-11). At times all three persons appear or are mentioned together (Matthew 3:16-17; Matthew 28:19; 2 Corinthians 13:14).
The doctrine of the Trinity means there is one God in three persons. The Trinity is mysterious and unique, because no object on earth compares to God’s Triune nature.
Because God is one, there is no division in God’s will or his attributes. When we speak of the Father, the Son, and the Holy Spirit, we are not referring to various degrees of divinity. All are fully divine.
Because God is three, there is relationality within God. The Father is not the Son. The Son is not the Spirit. The Spirit is not the Father. All are in perfect relationship with one another.
Is the doctrine of the Trinity a dusty doctrine of no relevance today? Certainly not. First, because God is Triune, he has been in relationship with himself eternally. He is perfect relationship and teaches us what relationship was always meant to be.
Second, love is eternal, because love always existed among the three persons of the Trinity. Pastor Kevin DeYoung said: “If you have a god who is not three persons, he has to create a being to love, to be an expression of his love. […] God didn’t have to go outside of himself to love. Love is eternal. And when you have a triune God, you have fully this God who is love.”