It was not the bible till man recognized it as the bible. Collections of old writings do NOT a bible make.
Let's see now...these old writings were authored by more than 40 different men, from all walks of life, on 3 different continents, in 3 languages, across about 1600 years in time, makeing collusion impossible...yet they tell one cohesive story. Further, many of the prophecies of the older writings have already been fulfilled. Archaeological evidence confirms the writings as well. The writings themselves contain claims to be the Word of God - claims which can be tested.
These are some of the factors that argue for the veracity of the writings and validate their collection into the book we call the Bible.