according to the revelation...? which was kept secret since the world began, Romans 6: 25 and Ephesians 3: 1-6 Which by the way explains through faith from by faith Romans 3: 30
Paul was getting no where fast getting converts in Tarsus.
The people there were mostly followers of Mithra. They believed that blood sacrifices washed away sins.
Paul blended this idea into his ever-changing version of Christianity. Only he supercharged it with the blood of Jesus (Son of God) washes away the sins of the world. Prior to this it had no part in Christian teachings. Jesus never said anything about it. It worked like a charm. Paul had a deluge of converts because He gave them something that they could relate to.
Constantine and his gang did not overlook this success Paul had with this blood sacrifices idea. They were trying to maneuver them selves into the control seat of the Christian faith. They adopted Paul's version of Christianity and the idea of God/Jesus dying for your sins. It was very effective because of the guilt it induced. They also took another hint from Paul's book of tricks and came up with a very similar version of Paul's conversion for Constantine. He too had a vision where the Christian Figure head came to him and converted him.
The rest is history. After the council of nicea all competing scripture was ordered destroyed and all true versions of the teachings of Jesus were labeled heresy.
All we were left with was what became known as the bible.