In answer to the OP, I repeat her what I said several months ago on this same thread.
The Trinity doctrine became established in the church through coercion from both church and state.
I think that any true doctrine that comes to light doesn't need the power of the state to convince the church of its validity. Benjamin Franklin spoke on this wise...“When a religion is good, I conceive it will support itself; and when it does not support itself, and God does not take care to support it so that its professors are obliged to call for help of the civil power, 'tis a sign, I apprehend, of its being a bad one.”
Compulsion and persecution over the Trinity doctrine became significant after Christianity became intertwined with imperial power. The state actively enforced doctrinal conformity, and those who dissented from the Trinitarian view faced legal and social penalties, and sometimes violence, especially from the late 4th century onwards.
Destruction of Arian kingdoms the Heruli, Vandals, and Ostrogoths were eventually destroyed or absorbed by forces loyal to the Catholic Church and the Byzantine Empire. Historical accounts suggest that the Papacy and its allies, particularly Emperor Justinian, played a significant role in orchestrating military campaigns against these kingdoms, motivated in part by the desire to eliminate Arianism and establish religious unity under the Nicene Creed. The destruction off those kingdoms matches perfectly with the prophecy of Daniel regarding the example uprooting of 3 horns by the little horn...aka the papacy. The other 7 kingdoms were also originally Arian for the most part, but converted under pressure from the Catholic church, particularly through the threat of war from, for example, Clovis, King of the Franks, who was later known traditionally, as a result of his conversion, often referred to as the “first Catholic prince” and later acquired the honorary titles of “Most Christian King” and “Eldest Son of the Church” (“Rex Christianissimus” and “primogenitus Ecclesiae” in Latin).
In saying the above, I am not discussing the Trinity as a doctrine of faith. I'm merely providing historical context as to how and when it became "Orthodox".
One question that could be asked, is it necessary to believe the trinitarian doctrine as per the teaching of the creeds in order to be identified as a Christian, if one fully accepts and believes in the preexistence of Christ, His divinity, and the divinity and personality of the holy Spirit?