Why do we believe in the trinity?

[Terrywoodenpic]

Actually, this is not the case. The Trinity was first elaborated in its current, full-fledged form by Eastern Fathers, particularly St. Athanasius of Alexandria. In fact, every Ecumenical Council was dominated by Eastern Fathers (The Pope of Rome wasn't even invited on one occasion!) and every solution to problems about theology and Christology came from the East, rather than from the West.

In fact, the First Council of Nicaea explicitly limited Rome's jurisdiction to Italy alone.

In summation, the Church of the first millennium was far from ever being "Romanized."

The First Council of Nicea was convened by Constantine the Great upon the recommendations of a synod led by Hosius of Córdoba in the Eastertide of 325. This synod had been charged with investigation of the trouble brought about by the Arian controversy in the Greek-speaking east.[16] To most bishops, the teachings of Arius were heretical and dangerous to the salvation of souls. In the summer of 325, the bishops of all provinces were summoned to Nicea (now known as İznik, in modern-day Turkey), a place easily accessible to the majority of delegates, particularly those of Asia Minor, Georgia, Armenia, Syria, Palestine, Egypt, Greece, and Thrace.
This was the first general council in the history of the Church since the Apostolic Council of Jerusalem, the Apostolic council having established the conditions upon which Gentiles could join the Church.[17] In the Council of Nicea, "the Church had taken her first great step to define doctrine more precisely in response to a challenge from a heretical theology."[18]

From WIKI

Constantine had invited all 1800 bishops of the Christian church (about 1000 in the east and 800 in the west), but a smaller and unknown number attended.
From WIKI

Though invited no bishops from Britain attended, In fact only something over 300 Bishops attended.(out of the possible 1800)

The Bishops Had their Stay and travel paid by Constantine. And were "Required" to stay until the issue was resolved.

Constantine needed the New state religion to be unified through out the Empire. All bishops of what ever Church East or west were invited , there were no restrictions.
Those that disagreed with the vote were declared heretic and banished. ( Principally Arius.)

 

[Terrywoodenpic]

The reference in the Didache Chapter 7 is pretty much unanimously considered to be an interpolation, and this formula is interpolated in other writings like the long form of Ignatius's epistles, and likely even Matthew 28:19 which differs from Eusebiu's account)

I would agree that many earlier writers did think as you say, but they assumed it was quoting a Trinity formula.

Aaron Milavec, the leading modern scholar of the Didache, notes that this usage is Not a formula, as in the modern baptism, but was using the Hebraic expression of acting" in the name of X" it has to do with the way a servant or disciple is authorised to act.... he go's on to explain this in detail ....It was simply a trinity of names.

The whole Didache is written as a Christianised form of the Jewish two ways of life and needs to be read in the Judaic-Christian Context, of the new Jesus movement.