Without citing the bible, what marks the bible as the one book with God's message?

[Mark 13:13]

But TBH we are encouraged to develop our knowledge of GOD by personal experience.

In other words, you're inventing what you want God to be. I do that when I write fiction, it's fun.

Well if by subjective experience and subjective insights what in the extreme the church considers and terms Private Revelations you wish to define as inventing then yes. I would have many scientist in my company.

defination
to originate or create as a product of one's own ingenuity, experimentation, or contrivance