Stepping back a bit from the considerations I have discussed in the last two days, I suggest that there are fundamental questions of ecclesiology involved.
Take the idea that the Church does well to be involved in Community affairs, and that this manifests the Incarnation of the Lord whose Body the Church. I don't feel that this is so much wrong as important but decidedly secondary. Surely, it is historically a working-out of the consequences of the Constantinian revolution, when the Church emerged (metaphorically) from the catacombs of persecution and walked straight out into Government favour and the possibility of changing Society for the better. It owes a great deal to conditions in late Antiquity, when ecclesiastical institutions to some degree occupied a partial vacuum left by the collapse of some imperial structures. But it does not form part of core ecclesiology. The Lord, after all, did not go around giving advice on the structures of secular life and how to improve the economy. He talked about the Kingdom and does not appear to have taught a Marxian Kingdom of this world.
I do not much believe in the notion that the Church is the only institution which exists for those who are not members - august though the proponent of the idea may have been - because I do not believe that the Church exists, primarily and in the last resort, to do good deeds in the world. With all due respect to Blessed Theresa of Calcutta, I notice that the New Testament invitations to fraternal benevolence relate primarily to the redeemed society itself ... "do good unto all men but especially to those who are of the household of the Faith". The heart of Christian ethics in the New Testament epistles is the relationship between those who are fellow members of Christ. Read Philemon, in which, notoriously, S Paul does not discuss the institution of Slavery and appears to have no awareness of the Rights of Man, but bases his entire casuistry on the transforming fact that this slave and this owner are both en Christoi.
I admire (and find myself judged by) those great Saints who, down the centuries, have displayed the unbounded love of Christ to men and women far beyond the visible boundaries of His Body. But fundamentally it is the Church which matters; and the purpose of the Church is to be Christ's one Body and to offer in all places from East to West one pure Oblation. It is of the essence that this Body should be one, as Christ is in the Father and the Father in Him.
That is why I feel so strongly the imperative to Unity. In an imperfect world, discipleship can indeed mean starting in the place where we were placed; it can mean joining with others in the discernment of the way ahead; it can mean making prudential judgements about timescales. But I need to be able to give a straight and honest answer to the Lord's question "Are you walking towards the oneness of My Body, or are you walking away from it?"
To comment, visit his fine blog Liturgical Notes.