Got it. I first thought that you were referring to a belief that all Christians have the same belief about those central metaphysical tenants and refuse to debate about what they believe is an empirical truth...likely like the creationist guy on the mountain with you.
I wholeheartedly agree with you and that's coming from someone who identifies most closely with Christianity. Something that drives me insane about fundamentalist and mainline Christianity is the approach that the Bible and its contents must be taken as a sort of empirical truth, completely disregarding the deeper meanings of the text that were never intended to be taken that way...not to mention completely ignoring the fact that the text itself is deeply flawed and full of errors, even further ignoring the fact that we don't even have complete manuscripts of any of it and the manuscripts we have aren't in sync. Debating the "literal truth" of that text and belief system is really, in my opinion, the most basic and most boring approach to a topic of shared human experiences that should be deeply rich; it's an approach that makes it a juvenile religion.
Which metaphysical claims of Christianity aren't up for debate?
And I say that as someone who recognizes the metaphysical claims of Christians aren't up for debate so have at it if you want to, but some stuff you can call bullshit on.
I would say the central ones, that Jesus was the son of God, came here to forgive sin, provide eternal life, etc. There's just no way to prove any of that one way or another.