There are “biblical” words that are not, in fact, in the Bible. For example, you will not find the word substitute in your Bible, and yet the word conveys the beauty of what happened when the Son of God died in the place of sinners. You will not find the word Trinity in the Scriptures either, and yet it represents true teaching about who God is. And the truth it captures is wondrous.

Adding to the wonder of the doctrine of the Trinity is the fact that such a teaching arose from a faith that was utterly committed to the one-ness, the unity, of God. The truth that there is one God was fundamental for Old Testament believers. The passage that articulated the monotheistic heart of the Hebrew faith, and which every Jew knew by heart, declared, “The LORD our God, the LORD is one” (Deuteronomy 6:4). Salvation is found in this one God alone: “Everyone who calls on the name of the LORD shall be saved.” It would be impossible to overstate how deeply the truth of God’s unity was embedded into the thinking of God’s old-covenant people.

This was true when the early church exploded into being in Jerusalem. For men like Peter and Paul, the unity of God was axiomatic. The only way for them to come to understand that the one God exists in three Persons, then, would be for that God to reveal Himself to them. This is precisely what happened in the person of Jesus Christ, who claimed authority to forgive sins and verified that authority by doing what is possible only for God to do (Mark 2:1-12). In this way Jesus revealed Himself to be the divine Son of the divine Father, and together They would send the divine Spirit to Their people (John 14:23-26). Then, before ascending to heaven, this Jesus commissioned His disciples to make disciples of all nations and to baptize them in the one name of God, which is attributed to three Persons: God the Father, God the Son, and God the Holy Spirit (Matthew 28:19-20). The disciples understood that the Lord of the Old Testament had been with them in Jesus. They had been fishing with the Lord, had sat at His feet, and had watched Him work wonders. And they understood that when they urged people to call on the name of the Lord to be saved, they were urging them to call on Jesus Christ (Acts 2:21; Romans 10:13).

For the disciples, this understanding came as the Spirit of God worked in their hearts and minds as they considered what Jesus had taught them and what He had done. For us, it comes as we meditate on the Scriptures and let the Spirit illumine its pages for us. Yet the truth of God’s triune nature is not for our mental exercise alone; it also ought to cause our hearts to swell in adoration of our vast, mysterious, and transcendent God, who nevertheless draws near to us. And it ought to fuel us, as it did the disciples, to go into this unbelieving world with confidence that this triune God—Father, Son, and Spirit—will draw people to Himself as the gospel message goes forth.