Christianity is false. Any law based upon it is therefore based on false premis

[The Mikado]

What does "false" mean under the circumstances? Whether God exists or not has little to no relevance to the question of whether the majority of people in society accept certain moral claims as valid. The existence of God is irrelevant to the question of whether or not each specific piece of Biblical law is worth preserving or not.

There's a fantastic story in the Talmud in which two rabbis are arguing over a specific point of law when God Himself comes in to tell them what He originally meant, and the rabbis chastise Him and tell Him that his role in the legislative process ended with the creation of the laws and that it's up to them, the rabbis, to figure out what the law means now.  It drives home the point that, regardless of the original provenance of a principle of religious law, once it is law, it belongs to humans, not God, and humans can interpret it and do what they wish with it.