It is a common assumption that the New Testament shifts the focus from Israel to the nations. The New Testament does give a clear directive to take the gospel to the nations and also describes the spread of the gospel among the gentiles. However this Great Commission is not disconnected from God’s promises to Israel.
Most believers assume that the Old Testament is primarily about Israel and the New Testament is a shift in emphasis in the nations as defined by the Great Commission. However, we must see the Old and New Testament as a single, unified story and the Great Commission as deeply rooted in both the Old and New Testaments.