"The NT writers didn't have Bibles and the concept of a faith based on writings (that the vast majrity of people were not equipped to read would have been bizarre to them."
"All Scripture is given by the inspiration of God, and is profitable for doctrine, for reproof, for corection, for instruction in righteousness..." --2 Timothy 3:16 This is, of course, the Old Testament, in particular the complete Septuagint.

"They also had no concept of the trinity ..."
Actually, they did. I point to you the writings o the Apostolic Father, such as Saint Barnabas and Saint Ignatius: "There is one Physician who is both flesh and spirit, born and yet not born, who is God in man, true life in death, both of Mary and of God, first passible than then impassible, Jesus Christ our Lord" --Ignatius to the Ephesians 7:2
I also point you to the apologia of Athenagoras the Athenian: "...And the Son being in the Father and the Father in the Son, in oneness and power of the Spirit, the understanding and reason (nous and logos) of the Father is the Son of God ... He is the first product of the Father, not as having been brought into existence. For from the begininng, God, who is eternal nous had the logos in Himself, being from eternity instinct with Logos ...