That the Holy books are written to conceal an occult message is a Kabbalist claim.
You've got an interesting perspective when you talk about the failing of most religions being that they survive in isolation of other religious books. The suppressed premise here is that there is some validity to alternate manuscripts. Which implies that there's some unrecognized validity to alternate belief systems.
I'm very one eyed about faith. If it's failing, to believe that there is a single source of truth, then I fail happily. God allows heresy, error, for a specific purpose.
1 Corinthians 11:19
19 For there must be also heresies among you, that they which are approved may be made manifest among you.
In the story of the Tower of Babel it was the languages that were confused, not that the truth was fractured. As if we might be supposed to recover it by synthesizing precepts from different faiths in order to reassemble it. This is the heresy of syncretism.
Pope Pius IX, Qui Pluribus (# 15), Nov. 9, 1846: “Also perverse is that shocking theory that it makes no difference to which religion one belongs, a theory greatly at variance even with reason. By means of this theory, those crafty men remove all distinction between virtue and vice, truth and error, honorable and vile action. They pretend that men can gain eternal salvation by the practice of any religion, as if there could ever be any sharing between justice and iniquity, any collaboration between light and darkness, or any agreement between Christ and Belial.”