That's a false assertion. People don't believe in all sorts of things that ARE real and they do believe in all sorts of things that are NOT real. You're working from a position of fallacious reasoning.
In any event, I'm not suggesting one must hold a Trinitarian view in order to be saved but rather that the reality of God's triune nature is essential to understanding who God is and therefore essential to the Christian worldview.
It's your choice what you choose to believe and how you choose to live your life even when you choose to believe things that aren't real, disbelieve things that are, and align with God's morality or against it.
As I stated, there is good fundamentalism (e.g. people who align with truth in the right way) and bad fundamentalism (e.g. people who do not align with truth or not in the right way).
But now we're going in circles because you are making false assertions one of which is engaging in relativism where no relativism exists.
It's hardly a false assertion when its taken in its own proper context. Both doctrines, oneness, and trinitarianism, have a wide following because both viewpoints can seemingly be taken from the bible's wording. You're convinced of a truine nature because you believe it to be true. By your own reasoning (that I'm making a false assertion on the nature of belief in regards to this specific topic), and by me looking at the bible, I could, from my standpoint, find you as someone who believes in a fallacy. From the horses mouth, here's just one reason why;
Look;
'There is one God, and also one mediator between God and men; the man Jesus'
Here's another;
'You are my son, whom I love dearly'
and another;
'Father, father, why have you abandoned me?'
The list can go on. And why in the name of God, would God name himself Jesus, become human, let mankind torture and kill him and talk to himself?
The point is, I'm not coming from fallacious reasoning regarding oneness belief, because its basis is right there, in the very book you read. The same book that you get your trinitarian ideals from.
Therefore, (moreso for your sake than oneness believers, and for the sake of persecuted people everywhere), I think it's reasonable to allow for varying interpretations. Even if I do find some of them patently ridiculous ....