A Because that is how God has revealed himself in his Word: these three distinct persons are one, true, eternal God.
This weeks question addresses the doctrine of the Trinity. The word trinity is a word that means tri-unity or three in one. It is not a simple concept to explain, but it is one that Christians profess. We believe that God is one, as the great passage from Deuteronomy 6:4 tells us, “Hear O Israel, the Lord our God, the Lord is one.” But we also believe that God has revealed himself in three different persons in the Scriptures. There is God the Father to whom Jesus prays, there is God the Son, with whom the Father is pleased and there is God the Spirit who is sent to take Christ’s place in the earth after his return to heaven. Read Matthew 6:9, 4:16-17 and John 16:13-15.
Read 1Corinthians 8:4-6. In this passage Paul speaks of the one God, but then he goes on to name both the Father and the Son. Both are ascribed the majesty of creation, one we live for, the other we live by. These two persons of the godhead are obviously viewed as equals, one and the same. They are the one God Paul speaks about.
The Great Commission is a wonderful passage not only for reflecting on our call to evangelize the world, but also for its theological teaching on the persons of the Godhead. We know that to worship anyone but God is to be guilty of idolatry. But in this passage, we are told plainly that the disciples worshipped Jesus. Either they were guilty of idolatry or they were true in their understanding that Jesus was God. Christ’s own words make it clear that they understood that God was triune. We are told to baptize and make disciples of God, the only God known by three persons: Father, Son and Holy Spirit. These three make up the one God who will rule into eternity. Read Matthew 28:16-29.
Paul’s closing words in Galatians are a wonderful benediction, but they also affirm the three persons of God. After arguing for salvation as a gift from God, and challenging false doctrine, would Paul throw it all away by a careless benediction that affirms some other God alone from the God of the Scriptures? Certainly not. And so he encourages us by referring to Father, Son, and Spirit, all of who are active in our life and salvation. Read 2 Corinthians 13:14.
In his letter to Titus, Paul affirms the deity of Christ and the ties the Holy Spirit directly into our salvation. Read Titus 2:11-14 and 3:4-7. The Father, Son and Holy Spirit are all named as equal participants in our salvation process and we know from earlier questions that only God can save us, no created thing has the ability to appease God. And so we are encouraged yet again, three exist and they are one God, as the Westminster catechism says, “the same in substance and equal in power and glory.”
For those desiring an Old Testament reference to the trinity, that is hard to do. But one of the classic passages which has been appealed to is the story of the three guests who come to eat with Abraham. Read Genesis 18:1-10. Many in the early church saw in the three guests a foreshadowing of the Trinity. However I prefer the passage from Exodus 23:20-23 where God clearly speaks of an angel of the Lord whom we must obey for God’s “name is in him.” This can refer to none other than Jesus Christ, God’s son.
For those wishing to explore this theme further, I recommend reading the Athanasian Creed, which sets out the doctrine of God in wonderful terms, without wading beyond into speculation.