This brings us right into the vexed question of the relationship between the two covenants – how did the new covenant inaugurated by Jesus relate to the law of Moses and God’s promises to Abraham [Gen 12:1-3]?
How do you ‘get in’ to this new covenant? What is required of those today who are the people of God?
For anyone reading books on Paul today will know that this takes us right into ‘New Perspective on Paul’ territory – a huge (and at times confusing) debate about what exactly Paul meant by’ justification by faith’. We’ll come back to this later in this series.
Bird does a great job of clear explanation without getting too distracted by detail. The big point here is how Paul’s interpretation of Abraham stood in contrast to popular Jewish rabbinical understanding. While in 1st Cent Jewish thought Abraham was held up as a paragon of faithful obedience to the law who was blessed by God as a result [work-for-reward theology], Paul uses Abraham to make a radically different point.
Abraham is talked of by Paul in Romans and Galatians as an example of someone who is accepted by God through faith. Salvation is not of works and is not limited to the physical people of Israel. Thus circumcision (the mark of a male Jew or of a Gentile proselyte who wishes to convert to Judaism) is not crucial for Paul. To make it so would be to make being Jewish essential to being in a right relationship with God – something Paul is adamant is not the case.
In this sense Abraham anticipates the gospel.
‘Abraham had faith in God’s life-giving power and so do Christians when they confess the risen Lord … in Galatians 3, Paul treats the Galatians receipt of the Spirit as equivalent to Abraham’s receipt of righteousness. It is by believing like Abraham that Galatians become the children of Abraham.’ 
The Law could not deliver such sonship, or the gift of the Spirit or righteousness but only curses for failing to obey it. It is Jesus who redeems believers from this curse [Gal 3:14]. Believers in Jesus are heirs to the promise of Abraham. In the new covenant there are no ethnic barriers such as those marked out by the law.
Bird points how Paul is reading the scriptures as a narrative – a story of salvation that points forward to Jesus. This is in utter contrast to a Jewish absolutisation of the Law and of Abraham. Paul is reading the story of Abraham as a model of faith whereas Jews of his day read Abraham in terms of physical descent (ethnic identity and circumcision and obedience to the law).
You can see how this set the scene for the divergence of Christianity out from Judaism. Paul is saying that Judaism rightly understood is not about physical descent, but trust in the God who made promises to Abraham – promises that include non-Jews.
Paul is not rejecting the law or rejecting his Jewish faith and heritage – he is reinterpreting what it means to be a child of Abraham in light of Christ.
In other words this is fulfilment theology – the new covenant is a fulfilment and culmination of the old.
Is this how you see Paul’s theology of Abraham? If not, what are the differences?
This of course has implications for how Israel is understood to fit within this big story. And it is to Israel that Bird turns next.