I'm not really interested in discussing the truth of religion itself, just the meaning of "forgery" and "authentic" in this context. You don't need to believe any of the religious claims to discus that.
The historical development of texts and how they are doctored is informative.
For example, early Canaanite texts assigned territory to tribes according to their number of gods. In a later text, it was by their number of tutelary angels. In current texts it is unspecified why they got their allocation.
Oldest Canaanite texts have a goddess Asherah, equal to YHWH in importance (and always shown with a cake), involved in important rites. Nowadays they use a stick in her place, which appears to suffice. They clearly would have preferred to do away with her entirely, but weren't willing to abandon the rites that needed her.
Religious rites were indistinguishable from magic spells, and the ancients didn't care who they came from, if they seemed to work. They were swapped around like coleslaw recipes. You can see how they were thought of particularly in Numbers, where it clearly doesn't matter who uses them: if you say it, it works, priest or no.
