Where did Unitarianism come from?

[Matthias]

I wouldn't have a problem with this statement.

It’s just a simple statement of historical fact. In and of itself, it doesn’t prove that trinitarianism is wrong.

To me, the Trinity is a NT revelation, blazingly clear, but only in the NT.

Have you read Edmund Fortman’s book The Triune God: A Historical Study of the Doctrine of the Trinity? If not, it’s a trinitarian resource which I highly recommend to all readers, trinitarian and non-trinitarian alike. It was in this book that I first came across the term “elemental trinitarianism”.

Questions about progressive revelation (which any adherent of the NT must logically adhere to), are a complex issue.

 

[Matthias]

Quick story. I was speaking with a trinitarian a couple of years ago about the Trinity. He agreed in the course of our conversation that the prophet Isaiah was a unitarian. However, he said that after “Isaiah the Unitarian” died and went to heaven he was surprised to discover that God is the Trinity!

Well, I asked, then what happened?

He replied, Isaiah is no fool. He converted to trinitarianism.

Voila! Thus dis “Isaiah the Unitarian” became “Isaiah the Trinitarian.”

I laughed heartily, but my trinitarian conversation partner was quite serious. He really believed, based on his Baptist faith, that this actually happened.

 

[Richard]

Unitarianism is the oldest and most lasting deviation from Christianity. A lot of people that believe in this one also believe in Universalism

 

[Pancho Frijoles]

Unitarianism is the oldest and most lasting deviation from Christianity. A lot of people that believe in this one also believe in Universalism

I don't believe so.
Strictly speaking, the idea that God was One Agent, One Single Mind, started with the ancient Israel, but in a stronger form with Zoroaster.
After the exile in Babylon, where the Jews were in close contact with God's revelation through Zoroaster, the Jews, who have fluctuated between considering god a single person or a class of persons, became firmly monotheistic.
That's why you see that Jesus spends practically no time or effort in speaking against gods from surrounding countries.

In regard to Christianism, remember that several ways to think about Christ coexisted in the first three centuries.
Unitarians would say that Trinitarism was a deviation from the teachings of Jesus and his apostles.