"In recognizing Jesus as Messiah (‘Christ’ in Gk.), i.e. as the one in whom God’s purposes for Israel had been summed up, Paul was compelled to rethink the place of Israel, and of her law, in God’s over-all purposes. Unless God had changed his plans (which was unthinkable), that which had happened in Christ must have been God’s intention all along. The cross and resurrection gave Paul the clue: since the Messiah represents Israel, Israel herself must ‘die’ and be ‘raised’ (Gal. 2:15-21). Reading the Scriptures again with this in mind, Paul discovered that, in the very passage where the covenant promises were first made (Gn. 15), two themes stood out: God’s desire that ‘all nations’ should share in the blessing of Abraham, and the faith of Abraham as the sign that he was indeed God’s covenant partner (Rom. 4; Gal. 3). But this meant that Israel’s understanding of her role in God’s plan had been wrong. She had mistaken a temporary stage in the plan (her land, her law and her ethnic privileges) for the final purpose itself. The law, however, although coming from God and reflecting his holiness, could not be the means of life, because of sin. But now Christ, not Israel, took centre stage: and in Christ, God’s plan for a worldwide family was being enacted. Israel’s political enemies were merely a metaphor or symbol for the real enemies of God, namely sin and death (1 Cor. 15:26, 56), which held sway over not merely Israel but the whole world.
These ultimate enemies had been overcome in the cross and resurrection. As the innocent representative of Israel, and hence of the human race, the Messiah had allowed sin and death to do their worst to him, and had emerged victorious. Sin’s power had exhausted itself by bringing to his death the one human being who, himself without sin, could properly be vindicated by God after death (2 Cor. 5:21). The cross thus stands at the heart of Paul’s theology, as the basis of his mission (2 Cor. 5:14-21), and of his redefinition of the people of God. The fact of universal sin (Rom. 1:18-3:20) demonstrates the necessity for a saving act of pure grace (3:21-26): the divine wrath (1:18 - 2:16) is turned aside, as at the exodus, by the blood of sacrifice (3:24-6). Had Israel herself not been captive to sin, covenant membership would have been definable in terms of law and circumcision: but in that case Christ would not have needed to die (Gal. 2:11-21). The resurrection provides the basis for the true definition of God’s people. God has vindicated Jesus as Messiah, and has thereby declared that those who belong to him, who in the Heb. idiom are ‘in Christ’ (cf. 2 Sa. 19:43 - 20:2), are the true Israel. The marks of new covenant membership are the signs of the Spirit’s work, i.e. faith in Jesus as Lord, belief in his resurrection, and baptism as the mark of entry into the historical people of God (Rom. 10:9-10; Col. 2:11-12). ‘Justification’ is thus God’s declaration in the present that someone is within the covenant, a declaration made not on the basis of the attempt to keep the Jewish law but on the basis of faith: because faith in Jesus is the evidence that God has, by his Spirit, begun a new work in a human life which he will surely bring to completion (Rom. 5:1-5; 8:31-39; Phil. 1:6; 1 Thes. 1:4-10). The present divine verdict therefore correctly anticipates that which will be issued on the last day on the basis of the entire life of the Christian (Rom. 2:5-11; 14:10-12; 2 Cor. 5:10). This double verdict is thus based on two things — the death and resurrection of Jesus and the work of the Spirit: Christ and the Spirit together achieve ‘that which the law could not do’ (Rom. 8:1-4). ‘Justification’ thus redefines the people of God, and opens that people to all who believe, whatever their racial or moral background.
The whole world is thus the sphere of God’s redeeming action in Christ, and men and women without distinction are summoned by the gospel to submit to Jesus’ lordship and so to enjoy the blessings of life in the covenant community, both in the present world and in that to come. God’s people form, in Christ, that true humanity which Israel was called to be but by herself could not be. Paul expresses this appropriately by referring to the church, the people of the Messiah, as ‘the body of Christ’ (Rom. 12; 1 Cor. 12). This membership in Christ must be lived out by individual Christians allowing the Spirit to direct their actions, enabling them to live in the present as appropriate for the heirs of God’s future kingdom (Rom. 8:12-25; Gal. 5:16-26; Col. 3:1-11). Since they are already thus participating in the new age, the final return of Christ may be soon or late, but should find them ‘awake’, not asleep in sin (1 Thes. 5:1-11; cf. Phil. 3:17-21). And when that day comes, not only human beings, but the whole creation, will share in the renewal which the one God has planned for his world (Rom. 8:18-25)." - NT Wright, New Dictionary of Theology