Paul here was writing to those who had already put their faith in Christ, yet were now estranged from him and his companions who were carrying the message of the Gospel. This fragmented relationship gave way to a disconnect from God's purposes towards them. To be estranged from God's ambassadors and community was to be estranged from God himself. Alternatively, to be reconciled to them was to be reconciled in Christ to God.
There is an urgency in Paul which invites his pleading with this fragmented community. Reconciliation and unity are not subsidiary realities to the Gospel, but at the core of salvation and what it means to be the church. If God was in Christ to reconcile the world to himself, and he is now in the church, then he must be at work within the church to reconcile men one to another.
God continues to work to reconcile the community of the redeemed and the church is still his chosen method in revealing himself. The message of salvation is incapable of being disconnected from its incarnation in the community of Christ. As Christ works through to us to plead to the world, he is also at work among us in a similar way with the plea, 'be reconciled to God,' and with it, 'be reconciled with each other.’
Source: A2J Community - Apprenticeship to Jesus Community, Phoenix, Blog Post "Unity Week Devotion - Day 5", 22 Jan 2016, http://www.a2jphoenix.org/blog/unity-week-devotion-day-5