Church unity and justification, is it illustrative or constitutive

Michael Bird comments

The error is frequently made of mistaking the context of justification with its purpose.There can be no doubt that justification and righteousness are rooted firmly in the debate concerning the identity of the people of God, but it is wrong to think that the verdict rendered in justification can be reduced to sociological descriptions of group-identity and self-definition.

[...]

Is the unity of Jews and Gentiles in the body of Christ illustrative of the effects of justification or is it constitutive in that God's saving righteous [sic] creates a new people, with a new status, in a new covenant, as a foretaste of the new age? I am persuaded that we must pursue the latter. Justification by faith means that God justifies the ungodly and calls our a new people to be the true Israel and the renewed human race.

(p.32-33, The Saving Righteousness of God: Studies on Paul, Justification and the New Perspective)

Mark Seifrid would disagree, and finds the unity of Jews and Gentiles illustrative...

The reason that the inclusion of Gentiles appears so regularly in connection with Paul’s teaching on justification is that their participation in the people of God was a visible expression of the justification of the ungodly (see Gal 2:11-21). Table-fellowship with Gentiles was therefore a call to mission, to the evangelization of the world, a call to an ever-expanding community. This community of Jews and Gentiles was not held together by any visible outward ties, but solely by the invisible bond of faith in the risen Messiah Rom 15:5-13). It was a community of forgiven sinners who came to one another not by means of an ideal of equality (defined on whose terms?), and certainly not by a common culture (cf. Rom 14:1-23), but through Jesus Christ alone. As Paul instructs his readers in Romans 9-11, Israel and the nations were, after all, God’s work. Their varying paths to Christ were the open, visible and necessary indications that God’s mercy, if it is to be mercy, must be radically free.

(The Narrative of Scripture and Justification by Faith: A Still Fresher Reading of Paul)