And when the Church positions itself as a microcosmic harbinger of the
eschatological new Creation that will some day displace the present
corrupted Creation in what you term "a complete ontological break," it
erects a strutural barrier between itself and the larger world in which
it is embedded. For those of us outside the faith and outside the
Church, this insular gospel is at best irrelevant to the common cause
of pursuing justice and peace for all in the present world.

The church, of course, isn’t obligated always to justify itself to those outside the faith or, for that matter, to pursue a ‘common cause’ of justice and peace. I would argue that the church stands in the first place for the ‘righteousness or justice of God’ in a rather broad sense: the God who desires justice and peace, to be expressed first within the people of God; the God who justifies a people that has rebelled against him; and a God who will ultimately be vindicated or justified as Creator by overcoming the wickedness, suffering and death that corrupt this creation - that is, by making all things new.

This distinctive God-centred rather than humanitarian mission is bound to some extent to alienate people who do not share its core presupposition. But I think that the primary calling of the church is to honour the creative God who, so the story goes, brought a creational microcosm into existence out of the ruins of the macrocosm for just that purpose. The church’s pursuit of justice and peace has to arise out of, and remain consistent with, that narrative. Everything we do is, at least potentially, a prophetic statement about God.

To the extent that the insular gospel deflects people’s zeal for
justice and peace away from the world and into the Church, it becomes
an obstacle to the common cause.

That’s an interesting remark. I can see the problem of the church being introspective and disinterested in global justice, etc. But I would still argue that it is more consistent with the biblical narrative for the church to pursue global justice by first demonstrating a true and comprehensive righteousness in its own internal life - by being that authentic microcosm. The church believes that the Spirit of the living God makes a difference to the business of being human. That implies boundaries and a willingness to protect our peculiar identity and vocation.