Trinity
It was several hundred years after the life of Jesus before many Christians accepted the idea that God was a trinity. It was a difficult idea because the Hebrew scriptures talk about God being One. The Greeks and the Romans could only understand Christ as a person who was bringing God’s word. It was not until the 4th century that the doctrine of the Trinity was fully developed in more technical and theological terms. This was established by the Council of Nicaea in 325 where, in the debate with Arius over the nature, incarnation and pre-existence of Jesus, the Son of God was explained as being of one substance (homoousios) with the Father. Christians had held to the tri-unity of God for centuries prior to this, and referred to Jesus as "God" and spoke in trinitarian language even before Nicaea codified the terminology (for example, see 1 Clement, the Epistle of Barnabas, the Letters of Ignatius etc.).