I would argue . . . it needs to be recognized that the Jew-Gentile controversy and tensions within earliest Christianity, occasioned particularly by Paul's mission, were not merely incidental to the emergence of the fundamental Christian doctrines regarding divine-human relationships but in substantial measure constitutive of them.
--James Dunn, "The Dialogue Progresses," in Lutherische und neue Perspektive: Beitraege zu einem Schluesselproblem der gegenwaertigen exegetischen Diskussion (ed. M. Bachmann; WUNT 182; Tuebingen: Mohr/Siebeck, 2005), 394; emphasis original
Dunn gives us two options as to the relationship of the horizontal to the vertical in Paul: "incidental" (referring to the "old" or "Lutheran" perspective) and "constitutive."
I reject both. A better word, between these two, is "derivative." The critical application of Paul's gospel in the first century was ethnic unity, the welcoming of Gentiles; but to say that this is the gospel confuses content with implication, essence with ramification, the "constitutive" with the "derivative."
Later Dunn writes, commenting on Ephesians:
Christ died to break down the wall, the law with its commandments and ordinances, the wall that divided Jew from Gentile. . . . The surmounting of these ancient hostilities was not merely a by-product of the gospel, far less a distraction from the true meaning of the gospel, but the climactic achievement of the gospel. (411; emphasis original)
No; Christ died to break down the wall between humanity and God. The climactic achievement of the gospel was and is new creation, restoration of Eden, reinaugurated fellowship between a holy God and wicked people. What Dunn says is of course true; but by calling horizontal unity the heart of the gospel he effectively so imbalances things that the net result of his incomplete truth is untruth.