The Princeton Proposal on Christian Unity, published in 2003 as In One Body Through the Cross has some bracing passages on the connection of unity and mission.
The authors insist that the common life of the church is not merely a means for mission, but an “essential goal of the mission that God has appointed for his people. Unity is not merely a means to mission, but rather a constituent goal: God gathers his people precisely in order to bring unity to a divided humanity.” Complacency or resignation in the face of the “wound” of Christian division is thus a turn “away from the mission God has given us” (26). We have to resist “consumerist values” and “an ideology of diversity” if we are to overcome the “ecumenical anesthesia” that is “one of the strongest present challenges to faithfulness” (43).
This unity cannot be invisible. As a part of mission it is mean to be seen by the world: “Invisible unity has no evangelistic power” (32).
The document also lays out the dangers of disunity. Quoting Paul in 1 Corinthians 1-2, it concludes that “When Christians are divided among themselves, each group must distinguish itself from the others by claiming its own special ‘strengths’ and ‘insights.’” This can only result in “boasting” in the specific goods of the group, emptying the cross of its power (33).
This as much as the “challenge of modernity” accounts for the anemic preaching, catechesis and discipline of the church: “The spiritual failure of Christianity in the modern era stems in many ways from our ongoing division” (33). Institutionalized divisions maintain themselves, and, worse, “breed never-ending diversion from authentic mission” as each group boasts in its “distinctives” as “something more unique than the gospel of Jesus Christ” (35).