That's an interesting thought experiment. I think the liberal Christianity you describe would operate like a secular humanist institution. The absurdity of liberal Christianity is when its members pretend they're not just agnostics or liberal atheists.

The problem is with organized Christianity, as an institution. Christ-like countercultures can exist, but not organized religions that pretend to uphold Jesus's anti-establishment values. That falsification would taint any such church.

And if Jesus is abandoned, why call the resulting institution "Christian"? What would distinguish this liberal religion from other religions? If the creed is viewed as a myth, then this religion would be a fan club, and its members would be appreciators of certain artworks.