If the community of God's people is the medium for communicating the message of God's good news, then many of the behavioral prescriptions of the New Testament are actually just as important in terms of their communicative properties as they are in terms of their ethical properties. What we do is important because God wants us to act a certain way, but it is just as important because of what it says about God's Kingdom...
In other words, we are commanded to behave in certain ways because God wants us to behave in those ways, but also because God wants our lifestyle to communicate certain things about the gospel. This is true in all NT commands but we will here discuss one in particular, and for two reasons. Firstly, the New Testament authors (Paul in particular) make it clear just how important this commandment is and are explicit about its communicative properties, and secondly, it is a particular shortcoming of the contemporary church.
I am speaking of the calling of the church to be the united, newly created, people of God, called out from among every ethnic group, every social class, every cultural group, and every language, into allegiance to Jesus.
John quotes Jesus' prayer for his followers, including those in the future (you and I). Jesus' prayer makes it clear that the unity of the church will communicate the veracity of the gospel to the world. In other words, when the world comes upon a community of people who have nothing in common (a community that crosses political, racial, language, class, educational, sexual boundaries) they will be forced to wonder just what it is that binds us together. Contrariwise, when the church ghettoizes itself along denominational, racial, political and socio-economic lines, we damage the proclamation of the gospel message.
"I am saved by faith, therefore____________"
The typical evangelical fill-in-the-blank answer to this is "I am not saved by effort," but this is not the scriptural answer! In every single place in the NT where justification by faith is brought up, it is tied directly to the unity of the Church across social divides. We have been blinded to the words of scripture, and have failed at our exegetical task, by the history of Protestant anti-Pelagianism. The Scriptural argument is "I am saved by faith, therefore there is no longer a division between Jew and Gentile, slave or free..."
Paul gets in Peter's face over this! This is not a peripheral issue. Paul essentially tells Peter he is denying the gospel when he refuses to eat with people of another race! This is a gospel issue for Paul, and should be for us too. Why are our communities of faith divided? Why do white Christians and Black Christians, English speaking believers and Spanish speaking ones, rich brothers and poor brothers, eat at separate tables? And even more importantly, why aren't we getting in anyone's face about this issue?
In short, you are not a committed disciple of Christ unless you are committed to work through your biases, and actively engaging in deep, formative, and submissive relationships with people who do not share your racial, economic, educational, political, or even theological, heritage.