Consider this: originally the Bible is the product of religious beliefs and culture, not the other way around.
It's rather silly to try and (re)create a religion based on a book that (incompletely) captures the religous thoughts and practices that 'inspired' it.
Of course the book is also he capture of a moment in time. The religious beliefs that are written down at one time continue to develop and evolve. An example of this is the evolution of God and morality from the OT to the NT.
So does it make sense that the religious community continued to develop their beliefs? Sure. So they added the trinity, an idea not yet captured in the Bible.
Then again, what exactly is the difference between the letters attributed to Paul, John, etc, and the letters and edicts somewhat later? They all simply reflect the religious thoughts of the day...