The Princeton Proposal for Christian Unity (One Body Through the Cross) works within two “poles”: On the one hand, there is an already-established unity, “a bond of faith and communion in Christ, established by the action of God.” On the other hand, we are called to “a deeper common life of reconciliation, mutual love, and shared labor.” The existing unity is “the necessary presupposition of ecumenical striving on our part” (section 8).
Ecumenical effort is thus the obedience of faith. It rests on the “conviction that the unitive power of the Holy Spirit, however inadequately respected in the present life of the churches, is already real and effective” (section 4). Given that the Spirit is at work to unify the church, “the quest for a deeper unity is internal to the life of faith.” It grows from the double conviction that “Christian unity is an intrinsic part of the transformed life God works among those who life in the faith of Jesus” and yet “it is a goal yet to be fully achieved in concrete, visible human terms” (section 5).This also means that refusal to pursue unity is unbelief and disobedience: “To work against the visible manifestation of the unity God has given to us, or to accept its absence with resignation, is . . . resistance to God’s Spirit and exposes us to God’s judgment” (section 6).
I would add only that the judgment isn’t just threatened, but already realized: Our divisions, partly the result of our indifference to division, are God’s judgment on our failure to live in the gospel.