"The “Secret Bible Knowledge” claim is very old."

You can say that again. It goes all the way back to the earliest foundations of Christianity when a group loosely labeled together as "gnostics" competed with other proto-Christians over the tenets of the faith.

For more than a century, there was no easily identifiable winner. The gnostics, who taught that Jewish scriptures and new teachings about Jesus contained hidden, mystical messages, were very popular, writing their own gospels to compete with those that eventually became Canon, and organizing apparently large networks of congregations across the eastern Mediterranean world.

One of their favorite books made it into the Canon and is now the last book of the New Testament.

The gnostics were eventually suppressed, often with violence, sometimes at official Church councils at the hands of hired gladiators.

The history of the early Church is surprisingly violent, actually, in terms of violence meted out by Church leaders. Lots of people died, for example, over minuscule differences in the definition of the Trinity.

I guess I'm writing this just to say that arguing over secret divine messages and picayune doctrinal differences is ridiculous, but all too historically common.