First Church Council of Nicaea
To settle ideological disputes in the Catholic (universal) Church, the newly converted Christian and Roman Emperor Constantine introduced and presided over the first ecumenical council at Nicaea in 325 A.D. In his book "The Heretics", Walter Nigg describes the means of reaching a consensus: "Constantine, who treated religious questions solely from a political point of view, assured unanimity by banishing all the bishops who would not sign the new profession of faith. In this way unity was achieved. 'It was altogether unheard-of that a universal creed should be instituted solely on the authority of the emperor, who as a catechumen was not even admitted to the mystery of the Eucharist and was totally unempowered to rule on the highest mysteries of the faith. Not a single bishop said a single word against this monstrous thing."
Nicene Creed refined the idea of the Trinity by negative pronouncements
The popular ideas of Tertullian and Origen were also superceded by the Nicene Creed. Tertullian taught that the divine Word (Jesus) existed originally within the Father's mind, and first became a distinct person when the world was created, and that the Spirit's personhood was subsequent to that of the Word. Origen thought of the Word as the offspring of the Father, and the Spirit as coming into being through the Word. He believed that the Son is god but only the Father is absolute God, or God in himself.
Both Tertullian and Origen, in my estimation, were truly spiritual men of God whose teachings should of been exalted instead of suppressed by the devolving Catholic church. Tertullian was a church father of Africa who, interestingly enough, thought of the orthodox Catholics as animal-men, whereas he considered his followers Spirit-filled. Origen distinguished between two classes of Christians; the simple, who are satisfied with faith in Christ crucified, and the perfect, who ascend beyond this to the contemplation of the Word dwelling with the Father.
The Trinity, a Catholic Doctrine
In 380 A.D. the emperor Theodosius passed a decree that read: "We shall believe in the single Deity of the Father, the Son, and the Holy Spirit, under the concept of equal majesty and of the Holy Trinity. 1. We command that those persons who follow this rule shall embrace the name of Catholic Christians. The rest, however, whom we adjudge demented and insane, shall sustain the infamy of heretical dogmas, their meeting places shall not receive the name of churches, and they shall be smitten first by divine vengeance and secondly by the retribution of our own initiative, which we shall assume in accordance with the divine judgment."
The Trinity is defined as being one God existing in three persons, all of one substance.