I have been reading and writing on this topic for some time. I haven’t yet published any of it. But, I will share a few tidbits now.
Emil Brunner rightly notes the church catholic is not properly an external institution or organization at all, but “a brotherhood resulting from faith in Christ.” The visible church is only the shell or instrument of this brotherhood. So, when we think of “the church,” we shouldn’t think,
Well, in a church you have the pastors, the deacons, and Christians who join the church. And, the whole Church is the collection of these individual churches.
That’s a corporate view; you look at God’s community as one big organizational chart with branches and sub-boxes for different denominations. This isn’t necessarily incorrect if you squint a certain way, but it’s not good enough. The Church, as Brunner says, is a brotherhood of all who have faith in Christ. It’s a far-flung family knit together by a love for one another that reflects God’s love. The immediate family is the local church. The extended family is the larger Church. You know some relatives better than others, but you’re all related. This picture of a far-flung family is better than an organizational chart.
What’s the mark of a true church? Christians have written about this for a long time, including me. But, again, these reflections often assume an organizational structure for the church. So, you have answers like, “a true church has apostolic doctrine, is holy, is one, and is catholic (that is, universal).”
But, carrying on the image of the Church as a brotherhood or family knit together by shared faith in Jesus Christ, why don’t we think of brotherly love as a defining mark of the redeemed community (1 Jn 4:19-21)? When it is missing, God is most angry (Jer 9:4-9; 1 Jn 5:1). Our brotherly love should reflect the intra-trinitarian, perichoretic love of Father, Son and Spirit (1 Jn 4:8). It is that love that binds them as One.
So, in a true “church family,” this love pushes outward, impelling the congregation to reach out to the world in love to offer them reconciliation, love and peace. It’s this love that moves us to evangelize, and it’s this same love that shows us as genuine to the lost. As Brunner has written, “[a] man is laid hold of by the life of the fellowship, moved by the love he experiences there; he ‘grows into’ the brotherhood, and only gradually learns to know Jesus Christ as the Church’s one foundation.
A church that fights is not attractive to outsiders, because it doesn’t reflect God’s love. Who wants to be a part of that? Nobody.
The one thing, the message of Christ, must have the other thing, love, as its commentary. Only then can it be understood and move people’s hearts. True, the decisive thing is the Word of witness to what God has done. But this Word of witness does not aim merely to teach, but also to move the heart.
A family that hates is not a functioning family. Likewise, a congregation that does not love one another is a dead church. There are other good marks of a true church. But, brotherly love must certainly be one of them.
 Emil Brunner, The Christian Doctrine of the Church, Faith, and the Consummation, Dogmatics vol. 3, trans. David Cairns (Philadelphia: Westminster, 1960), 128.
 Ibid, p. 129.
 Ibid, p. 136.