As a thought experiment, imagine a person with considerable wealth and/or vast unfettered access to professional printing services, to the point where this person can print up professionally-bound books of any desired content. This person prints up a (nearly) word-for-word copy of the New International Version of the Bible, with a few minor changes to Chapters 1 and 2 of the Book of Genesis, such that creation takes seven days and…
*Gen 2:1 Thus the heavens and the earth were completed in all their vast array.
2:2 By the eighth day God had finished the work he had been doing; so on the eighth day he rested from all his work.
2:3 Then God blessed the eighth day and made it holy, because on it he rested from all the work of creating that he had done.*
This would not be difficult at all to compose. Perhaps the creation on the fifth day, which in the NIV described God creating sea creatures and birds, gets spread out so on Day 5, it’s fish, and on Day 6, it’s birds, to be followed on Day 7 by land animals and humans, and Day 8 to rest.
Now, that person begins printing and distributing his new bible, handing it out for free, sending them by the tens of thousands all over the world without fanfare. Biblica incorporated may (and probably would) object as they are the copyright holders on the NIV, but beyond that what argument could anyone make that the seventh-day version is better than the eighth-day version? What evidence can they bring to bear, even hypothetically, that supports such a claim? And suppose a child in Peru or Gambia of Tasmania gets one of the new bibles as their first bible and accepts the eighth-day creation. What arguments can use to convince them they are wrong, assuming they are wrong?
In contrast, the person might also produce, say, a chemistry textbook which is in all respects identical to a standard textbook, except that in the new book’s periodic tables, gold is element #80 while mercury is element #79, a reversal of the standard. Now, objectors have an avenue of evidence - they can subject gold and mercury to various experiments (indeed recreating the experiments that placed gold and mercury in the periodic table originally) and demonstrate that the new chemistry text is indeed in error.
In general terms, which beliefs, if any, within the western religions (by which I mean Judaism, Christianity and Islam, and if anyone can suggest a better term to cover the faiths that revere Genesis, they are invited to do so) are not subject to casual substitution? If one produces a bible in which “God” is replaced by a pair of cooperating gods who always work together and are never mentioned separately, and all text accordingly altered ("… have no other gods before us…"), does that bible have more objective merit or less objective merit than a standard version?
And even more generally, what aspects of religious mythology are not arbitrary? If all bibles were lost, what process could one hypothetically use to recreate one, as one might use a scientific process to recreate the periodic table if all chemistry textbooks were lost? If the recreated bible has a few minor differences (or a lot of minor differences, or a few major ones), how could one prove it?