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But this is true of all preserved documents from antiquity so I do not see how once can claim that scriptures are “forgeries”

Because you don’t try, and you don’t seem to know much about how historiography works. If all documents from antiquity were equally unverifiable, then there would be no reason for getting to know ancient languages, carbon dating, understanding the context something was written to determine if they could have known what they are writing, and all the other things people do to determine what is more probably true. You are just repeating what someone told you without thinking about it.