It’s Yahweh’s laws but the mythology has it provided by Moses in his sermons to the Israelites. As for Christians ignoring bits of it, part of that is based on saying attributed to Jesus in the gospels (e.g. the bit from Mark I quoted above) and also the simple fact that most religions update themselves as society changes. If anything, I think the Catholic church was smart to have a leader who could receive “new revelations from God”. It lets them update canon, while maintaining the illusion that they aren’t just making shit up to stay relevant.