“In the beginning God created the heavens and the earth” is the very first verse in Scripture. If you start reading the Bible and never finish that is one truth of the Faith you will walk away knowing. Scripture begins with a monotheistic presumption. The apologetic is not to establish God’s existence but his identity. Throughout history God reveals his identity. The national cry of Israel, the Shema in Dt 6:4, would remind them of God’s oneness, “Hear, O Israel: The Lord our God, the Lord is one.” Progressively God reveals more of his identity as the mysteries of Scripture unveil him. Understand that when God gives a new revelation, it does not contradict a previous revelation. Revelation shows more clearly, what has already been revealed.
The New Testament writers begin with a monotheistic assumption, like Moses. They are not writing to establish the existence of God but his identity in Jesus. Moses and John both wrote to the people, to reveal to them the identity of the God that saved them. Therefore John writes “In the beginning was the Word, and the Word was with God and the Word was God” (Jn 1:1). Keeping their monotheistic convictions the writers must affirm that Christ was God dwelling with his people. The mystery is unveiled more as we understand that the Word is God and a distinct person from another person that is God. The mystery also unveils that the Holy Spirit is also a person that is God. In Romans 8 we understand that the Holy Spirit intercedes “himself” (8:26-27), gives life and raised Jesus from the dead. These are by no means exhaustive, nor meant to be, but the point should be taken that Scripture makes it clear that there is one God who has revealed himself in three persons, the Father, the Son, and the Holy Spirit.
The early churches soon found controversies in the ways that some would try to explain the apparent tensions that existed. Some proved helpful and some proved to be very dangerous but helpful to draw the lines clearer to understand properly how God was revealed and what contradicted earlier teachings. One of the first of these was “modalism” (Sabellius). It affirmed that God was one, however they also affirmed that God was one person who changed “modes” through history. In their view, God was Father, then Jesus, then (now) the Holy Spirit. One god acting in different “modes” in time. Adoptionism was another thought seeking to preserve the unity of God while holding in esteem the person of Christ. Adoptionists maintain that God created the Logos first, making him preeminent in creation. The Logos becomes incarnate and then because of his obedience as Jesus, the Father adopts him making him the Son of God. He is elevated and therefore is like God but he is not God as the Father is.
This epoch of errors culminated in the Council of Nicea AD 325. They produced the Nicene Creed affirming Christ as “the only-begotten Son of God, begotten of the Father before all worlds; God of God; Light of Light; Very God of Very God; Begotten, not made; Being of one substance with the Father, by whom all things were made” The idea affirmed was God was three in person and one in essence.