In John 17:21 the Lord prayed “that they also may be one in us: that the world may believe that thou hast sent me.” He prayed for the generations of Christians who would follow that first generation. v. 20 It was Jesus' prayer that there would be such unity among believers that the world would know God the Father sent God the Son v. 21, and that God has loved them v. 23.
This intended unity was not some abstract unity enjoyed in an invisible church… it is real unity between believers within local churches. To be with fellow Christians in a conference for a few days of rejoicing is wonderful, but it is not where real unity is hammered out. Real unity is hammered out in the day to day, up and down relationships that take place where people live and worship together over long periods of time.
This is what Paul wrote about in these passages in Romans. He moved the words of Christ from the abstract to the practical. We must learn unity in our local church… or we will not learn it at all.