Is the Bible a Credible or Authoritative Reference?

Whenever engaging in conversation with someone with orthodox Christian beliefs, I seem always confronted with their being stuck on the unquestioned idea that the Bible is the final word on spiritual understanding. My countering argument is always the same: There were no eye-witnesses living, remembering and passing along the Biblical stories and parables, allegorical stories that were much later collected and codified by the church, actually, hundreds of years later, and stories got changed, embellished, and edited all the time while being passed down from generation to generation. The Bible was based on information no one personally experienced or remembered. Time has a way of blurring history. Add to that the fact that the Roman Emperor Constantine, a non-believer, was the one who decided what stories would and would not be included in the Bible through the Councils of Nicaea 325 years after the time of Jesus for very political reasons, my instincts advise me not to extend the written Bible the credibility and authority that others give it.