How can a Christian reject part of the Bible and still call themselves a Christian?

[FallentoReason]

Law comes from morality, at least philosophically speaking.

-still wasting our time.

Moving the goalposts.

When push comes to shove, I actually identify as a contractualist (as opposed to Kantian ethics). In a nutshell, something is a moral rule only if no one has a legitimate objection to it, with objections needing to address why/why not someone's wellbeing is being violated. This mode of thinking captures the essence of debate, in which a conclusion is assumed once no one has a reasonable objection left to say. And if there's no objections, then to my mind it seems like that particular rule makes for a good law - something to be upheld globally.

-which has nothing to do with divine moral authority, and in fact if this were the moral setting schema it would prevent there from -being-  a divine moral authority even if there were some example of divinity floating around...which there isn't.

Err... why? If something has no valid objection to it, then it must be perfectly logical. If something abstract is perfectly logical, then it's akin to physical laws etc, ergo it could be a moral law set in place by a god.