progressive revelation my friend,
the bible always teached one God, in the old as well as in the new. describing the nature of God was at the time not usefull knowledge for them to know so i don't see why prophets of an old convenant allready had to believe the new before it was made, all thou the old testament is full of references to a triune nature in Psalms, Isaiah and Genesis it's never revealed in full context for obvious reasons that it would be ununderstandable without Jesus having come first...
"Your father Abraham rejoiced to see my day: and he saw it, and was glad." (John 8:56)
"Jesus said unto them, Verily, verily, I say unto you, Before Abraham was, I am." (John 8:58 )
Well that doesnt make sense, Jesus clearly says he did not come to abolish to old teachings, Think bout it, if you were a jew back then, and all of a sudden you start reading the Trinity THeory, they will call that blasphemy, There shoudlnt be this new God theory AFTER jesus, this all would of been settled if Moses and Abraham claimed and preached the Trinity, put urself in a jewish position