The Reformed faith is framed by covenantal thinking. The covenants of Scripture are what you see when you take away everything related to Reformed faith and practice. At the foundation of our doctrines is the doctrine of the Trinity. We don’t look at the Trinity as one of the doctrines that are added on or part of another doctrine. The doctrine of the Trinity serves to structure all of our faith and practice. It’s all over our theology. It’s everywhere in worship (praying, singing, preaching, sacraments). And the Trinity is part of the life of the Christian. Herman Bavinck stated “The Father, the Son and the Spirit is above us, before us, and within us.”
In history, God reveals himself to Israel and they will agree, as part of their covenant, that they will serve God and him alone. God is one and he says in many places of Scripture that there are no other gods besides him. Part of conversion and repentance of the people who turned to God was that they would have to give up their false idols. By nature humanity had known that God existed and they knew the true God. But their sin kept them loving and serving idols rather than God. It was only when God worked in their hearts they could love him.
Paul reaffirms that the Christians are not believing in a different God but the God of Israel.
“But this I confess to you, that according to the Way, which they call a sect, I worship the God of our fathers, believing everything laid down by the Law and written in the Prophets, having a hope in God, which these men themselves accept, that there will be a resurrection of both the just and the unjust.” (Acts 24:14–15 ESV)
The continuity is that the God of the Christians is the God of the Old Testament. Yet there is a distinction in the way that the Christians will look at the Old Testament. Now they will look at the Old Testament and see how it was always pointing to Christ. He was in every aspect of the Old Testament from Genesis to Malachi. He was the one promised in Gen 3:15
“I will put enmity between you and the woman,
and between your offspring and her offspring;
he shall bruise your head,
and you shall bruise his heel.”” (Genesis 3:15 ESV)
Jesus is the promised one to bruise the head of the serpent and have his heel bruised. And he is in the many other pictures of the promised messiah. Some were more explicit than others but he was always there. In a technical sense it is the Son of God who is there in the Old Testament because Jesus wasn’t here as a human till he was born of Mary.
It is in the New Testament we begin to see the fuller picture that God wanted to reveal; one God in three persons. The three persons are manifested in different places, sometimes alone, or together. In the baptism of Jesus, as part of blessings and greetings they’re mentioned everywhere. Each person of the trinity is also identified as objects of worship. They are worshipped as God and by Jews who were strictly monotheistic. Jesus is described in a remarkable way by John in chapter 1 as being co-eternal with the Father and part of creating everything that was made. The way the New Testament writers write about Jesus make it clear that he is more than any prophet Israel had experienced. He was God. The Spirit is woven through each of the Gospels in a way to show he is present and at work but his arrival in Acts is the arrival in power.
The Trinity ties all of Scripture together. This doctrine is why we read the whole Bible with a Christ-centered lens. It is one of the unique aspects of Reformed theology that frankly, shouldn’t be unique to Reformed theology. This is why Christ-centered preaching is another unique aspect of Reformed theology that frankly, shouldn’t be unique to Reformed theology. But all of this flows from that foundational principle of the doctrine of the Trinity. It’s simply everywhere in Scripture and therefore everywhere in our theology and practice.