Bruce Marshall has a lovely essay on the Church’s relation to Israel in The Cambridge Companion to Christian Doctrine. Let me summarize the highlights:
1. Marshall, like most Catholic and mainline Protestant theologians nowadays, thinks we should abandon supersessionism (= the proposal that the Church has replaced the Jews as God’s elect people).
2. He thinks this, principally, because supersessionism makes a God a liar (88). That is, if God promised an everlasting covenant with the biological descendants of Abraham (Gen 17:7), and if he then revokes that covenant in favor of a new one with the Church, then he lied to Israel; and in that case, what’s to keep him from lying to the Church? Another stream of Christian theology has argued that faith in Christ (as coming Messiah in the OT, as the Messiah who had come in and after the NT) was always the criterion of election (85); but this proposal sits uneasily alongside the Torah’s insistence upon the biological basis of Israel’s election; it is Abraham’s descendants through the line of Isaac who compose Israel.
3. If God’s promise to Israel is irrevocable, then “the salvation of the Jews is in the bag” (89); the only people in the world about whose salvation we need not worry are Jews. Marshall doesn’t draw out the most interesting consequence of this view, however, which (it seems to me, at least) is that, blaspheme though he might, a Jew can’t remove himself from the covenant. As Paul puts it, “All Israel will be saved” (Rom 11:26). Jews, then, are iconic demonstrations of the doctrines of irresistible grace and of the perseverance of the saints.
4. Point Two doesn’t entail that Jesus is not the Messiah of the Jews (90). If salvation comes through Jesus alone, then that goes for Jewish as well as Gentile salvation.
5. If Jews are to remain a unified people set apart from the Gentiles gathered to Christ, then the continued observance of the full Law must be acceptable (91-92). This does not run afoul of NT polemics against treating the Law as a means to salvation, because Jewish law observance is not typically understood in that manner; Marshall doesn’t go this far, but there’s a fair argument to be made that confusion about the saving power of the Law emerged principally as a debate within the community of early Christian Jews, whose Law observant mission to the Gentiles broke open difficult questions about the nature and sufficiency of Christ’s work in relation to Torah.