In my fundamentalist upbringing, people would bring up the "divine mystery" of the Trinity as a kind of proof of the truth of Christianity. As in, the fact that the Trinity cannot be explained must mean that it is beyond our human comprehension, and if it's beyond our comprehension, it must be divine.

But like, it's very easy to see how humans could create the idea of the Trinity, since it's simply asserting that multiple contradictory things are all true at the same time. Is God the Father separate from Jesus His son, or one and the same? Both, actually!

Plus, zealots in the church loved "uhm akshully"ing anyone who tried to use a metaphor to explain the Trinity. "The Trinity is like... water, and how you can find water as ice, water, and water vapor in different places." "UMM actually that's Modalism, and that's heresy!"

Basically the church just assigns an "-ism" for every conventional way to understand or know the Trinity, then insists that it is Unknowable.