When is a Religious Belief Delusional?

[Mister Agenda]

What do you mean by a mathematical object?  That's a term of art, and it's not clear what your question is.

I say that triangularity would exist in potential even if the physical universe had never come into being to manifest objects that sentient organisms would recognize as triangular.  Is my belief in triangularity wrong or delusional?

It's conceptual. It's an abstraction. It (the concept of triangularity) is not wrong or delusional in itself. But clearly, if the physical universe had never come into being, there would be no triangularity at all, nor any potential for such. Not only would there be no one to conceive of abstracting the properties that we call 'triangular', there would be nothing to observe to abstract those properties from.

Concepts require a conceiver.