Some years ago, around the time of widespread changes to liturgy, I came across a cartoon of a man in church turning to his neighbour and saying “When we used to say ‘We believe…’ I thought we all believed the same thing but now we say ‘I believe…’ I think we all believe something different.”
The problem with saying Jesus knew the Old Testament is that it didn’t actually exist at the time. The Jews all ‘knew’ which books they regarded as scripture but as it hadn’t been formalised there is more than a suspicion that everyone had different ideas about what they meant. However because ‘everyone knew’ and most of the Jews were still in Palestine there didn’t seem to be any reason to define what they meant.
This all changed after the Second Jewish Revolt against the Romans (or the Third Jewish Revolt depending if another revolt is counted as the second) in AD 132–136 when the Jews were dispersed from Palestine and it became important to define what books ‘everyone knew’ to be scripture. Since this is around 100 years after the death and resurrection of Jesus it is possible that the books Jesus regarded as scripture during his earthly life are not the same as the ones we have in the Old Testament (with or without Apocrypha.) This could explain why Jesus (and Paul) appears to quote scripture that isn’t in the Old Testament, he was quoting what he believed to be scripture but that hasn’t made it into our Bibles!
Now this becomes a big problem if you believe the Bible to be the literal, infallible Word of God as instead of a set text we have something of a more fluid nature. However viewed as a book telling God’s story and the story of his evolving relationship with mankind it doesn’t matter – what matters is the book we have now. Let me explain that point – I believe we have the Bible that God intended us to have; the right books with the right text we need in them (I’m thinking about things like the end of Mark’s Gospel and the story in John’s Gospel of the woman caught in adultery neither of which seem to be original.)
So do we have the Old Testament that Jesus knew? It seems unlikely but it doesn’t matter if we read the Bible as God’s story and not ours.
I believe that God the Holy Spirit can speak to us through the Bible in ways we cannot understand or foretell, surprising us and pointing us to Jesus, God in human form, and guiding us to live our lives like His. It is the book God intended us to have telling the story of His relationship with the Jews leading up to the climatic moment of the crucifixion. In view of this I believe that reading and studying the Bible are an important part of the Christian life.