Principle: The Trinity of God is the summit of all revelation.
So we have to speak of the Trinity in a way that faithfully conveys the idea of it being the greatest thing that could be revealed. But if modalism/ Sabellianism is true, then, at its heart, the revelation of the Trinity is nothing more than God does three different things. But we need no revelation to know this, so it makes no sense to call it the greatest possible revelation. Again, if Arianism is true, then the revelation of the Trinity amounts to saying God has a special relationship with the most special creature; but this is a tautology, and so we obviously need no revelation to know it. (We can leave off a discussion of Tritheism, since it is not a doctrine anyone advances but only an accusation of the ineptitude of another’s belief.)
So what about utter mysterianism? First of all, the name is inept – what one means to say is that the point of the revelation of the Trinity was to show that God was wholly unknowable. But this seems to be an even worse option than the other two since the whole point of revelation is to make something known to another. If the point was simply to convince us of ignorance, why bother saying so much about the Son and the Holy Spirit? Either the revelation of the Son and the Spirit tells us something about God or it doesn’t; if so, then mysterianism is false; if not, then the revelation is false – though one would only posit mysterianism to explain the truth of some revelation and so, on this second option, mysterianism becomes superfluous. If we say there is no Son of God, we need no theory (whether mysterianism or any other) to give an account of what role the Son of God might play in the life of God himself. Muslims or Buddhists are not mysterians about the Trinity, they simply do not assert that such a being was revelaed, and therefore have no need for a theory of the revelation.