The Gospel Coalition recently featured a blog post by Southern Seminary doctoral student David Schrock. In the post, “Beware in Your Search for Perfect Peace,” Schrock considers several qualifications for church unity. He writes:
While Christ prayed for unity, he also prayed for the sanctification of his church (John 17:17). The need for sanctification assumes the presence of sin, and sin’s presence promises disunity. Jesus knew this and did not pray naively. He prayed knowingly for immature churches, persecuted churches, proud churches, churches filled with sinful people-churches like the ones Paul addressed and Jesus rebuked in Revelation 1-3. His prayer is what glues sinners together, and it is by design that conflicting sinners would dwell together.
The Gospel Coalition provides the entire post.