First Council of Nicaea

In the eastern Roman empire a dispute broke out between Alexander, Bishop of Alexandria, and one of his presbyters (Arius) over the relation between the “Word” or “Son” of “God” which had been incarnate in Jesus, and “God” himself, now called “the Father” – his name, Yahweh, having been generally forgotten.
Practically all Christians now worshipped “Jesus, the Son of God” as a god, and would not consider giving up the practice.  Were there then two gods?  Or were the Son and the Father somehow one? 

Arius took the first position, making the Son an inferior god created by the Father.  Alexander excommunicated him, but many neighboring bishops supported him.  Emperor Constantine summoned all the bishops of the church to a council in 325 A.D. at Nicaea (present day Iznik, Turkey).  Some three hundred showed up and adopted a creed to which all “orthodox” Christians were thenceforth required by the state to assent.  The creed asserted belief in God the Father, and in Jesus Christ the Son of God “of the same substance as the Father.”  All bishops were willing to sign this, except two who were deposed and banished, as was Arius.  The new creed was enforced by the imperial government. 

Later in 381, the First Council of Constantinople declared the Holy Ghost of the same substance as the Father and the Son and so completed the official trinity, “one God in three persons.”