1. Treasuring the gospel
2. Living the kingdom
3. Embodying reconciliation
These are, I think, the main directives of the Church. We are to consider the announcement of the good news of Christ's finished work for the salvation of the world as "of first importance," letting it stir us to awe and gratitude and worshipful discipleship. It is the A-Z of our lives. Secondly, in our worshipful discipleship we live out in community the reality of God's kingdom being "at hand," with all that entails (the Sermon on the Mount is a great blueprint of kingdom life). And thirdly, we are missionaries -- ambassadors, "sent ones" -- carrying this gospel and living this kingdom for the benefit of a lost world and the lost people in it.
The way these things are carried out may vary in cultures and contexts, but these are, I think, non-negotiables for missional Christianity.
I see these three marks of the missional church in Jesus' High Priestly Prayer in John 17. Jesus prays:
"But now I am coming to you, and these things I speak in the world, that they may have my joy fulfilled in themselves. I have given them your word, and the world has hated them because they are not of the world, just as I am not of the world. I do not ask that you take them out of the world, but that you keep them from the evil one. They are not of the world, just as I am not of the world. Sanctify them in the truth; your word is truth. As you sent me into the world, so I have sent them into the world. And for their sake I consecrate myself, that they also may be sanctified in truth.
"I do not ask for these only, but also for those who will believe in me through their word, that they may all be one, just as you, Father, are in me, and I in you, that they also may be in us, so that the world may believe that you have sent me."
Where are the three marks?
1. Treasuring the gospel
Jesus begins by praying for his followers' joy -- actually, the joy of himself transferred into his followers, fulfilling their longing for lasting joy. Jesus connects this to the word of God given to them -- which is joyous in one important sense because Jesus the Messiah arrived after 500 years of prophetic silence and joyous in another because, as David says in the very first Psalm, the authoritative word of God is a delight to worshipers. And Jesus himself IS the authoritative Word of God.
Jesus also prays that the Church would be sanctified by this word, the word of truth. Cleansed by the proclamation of God based on Christ's self-consecration and atoning work -- Christ being the Word himself (John 1:1) and the Truth himself (John 14:6) -- we are to be joyful worshipers of he who is the Truth who rejoice in the word and living reality of the gospel.
2. Living the kingdom.
Jesus refers to kingdom living when he prays about his followers being hated by the world because they are not of the world (just as he is not) and when he prays that we will be protected from the devil. He also prays specifically that we won't be taken out of the world (at least, not yet), but that we would be sanctified while within the world.
This tells us two things, albeit implicitly: The Church is to live right in the midst of the world while not looking exactly like the world, and the Church's mission and consecration to God are (necessarily?) characterized by the evil one's attacks and the world's hatred. In other words, living as if Jesus is King and his kingdom is real and present will provoke the evil one's schemes and the world's persecutions. But this is direct Sermon on the Mount-type stuff.
3. Embodying reconciliation.
Jesus prays a beautiful, stunning prayer: "I do not ask for these only, but also for those who will believe in me through their word, that they may all be one, just as you, Father, are in me, and I in you, that they also may be in us, so that the world may believe that you have sent me."
Jesus isn't just praying for himself and the Church, he is praying for the "future church," for the world of prodigals waiting for the Church on mission to bring the good news to them, to be "ministers of reconciliation" (as Paul says) in both word and deed. Jesus speaks to the heart of the gospel here: reconciliation. Because broken relationship is the worst of the fallout of the Fall, because being restored eikons means being restored into reflection of the perfect unity of community in the Trinity ("Just as you, Father, are in me, and I in you..."), Jesus prays for the Great Commissioning of the Church to rep the gospel near and far in the hopes of reconciling people to God and thereby creating the reconciling people of the Church.
All for what reason? "So that the world may believe that [God] has sent [Jesus]."
John 17:1-5 sets that up more explicitly.
The good of the world is the glory of God and Jesus' prayer for the Church, present and future, is that we would undertake all aspects of our mission for that purpose.