Reggie McNeal, one of the current gurus of the missional church movement, estimates that a Google search of the word missional will yield over a million hits. That's big folks! (Sarcasm intended)
Bigger, however, is the question of whether missional, as presently expressed by its proponents, is biblical - or wise. Let me consider each of those issues.
First, I need to be clear, Jesus gave the church the mission of making disciples in all the world (Matthew 20:19; Mark 16:15; Acts 1:8 et al.). That commission is echoed in Paul's writings, for example II Corinthians 5 where Paul refers to himself (and by extension all believers) as ambassadors of God.
That commission was not new. The people of God (Israel in the Old Testament) have always had that mission. In the Old Testament Moses tells the reconstituted nation in Deuteronomy that the laws God had given them (and their living by those laws) would be an attraction to the surrounding nations who will ask where such good laws came from (Deuteronomy 4). More than that there was the provision and the expectation that some not of Israel would come and unite themselves with Israel, trusting in Yahweh as Israel did. Rahab, Ruth, Naaman and many others are examples of some who did. So mission is a basic default position of God's people, both in the Old Testament and the New. But is the present expression of it biblical? That is the question.
The present expression of missional places emphasis on going and on incarnational. In non-technical language, those mean being intentional about making disciples and to do so beyond the walls of the church building or her programs by living as well as speaking the good news that is the Gospel. All of that is good and biblical. And it is not new. (If you are interested this page is a good explanation of missional Friend of Missional)
Where missional departs from the biblical mandate in practice is in the neglect of the church for the mission of the church. (A careful reading of the page linked above suggests the "community" is crucial to the mission for the missional church, but it seems to me practice is otherwise.) Alan Hirsch, another guru of the movement, expressed it this way, "The mission has a church," (Defining Missional) in contrast to "The church has a mission."
Hirsch presents these two ideas as a dichotomy in which one is in contrast to the other. In the first, mission is logically prior to the church. That means mission is more important than the church, and the church is simply the current means by which God is calling people to himself. In the second, the church is logically prior to the mission. That means the church is primary and has a mission, perhaps one among several, that is secondary to the church. The latter idea expresses the present model which is much lamented in the missional church movement, the attractional church. In practice, however, the choice of the former over the latter has resulted in the neglect of the church in favor of the mission of the church.
I would suggest this is a false dichotomy. There is another option. That option is to recognize that the followers of Jesus are collectively his body (I Corinthians 12:27). Each has gifts and a calling to serve as a member of the body. Some of those gifts are designed to build up the body and some are designed to reach out to those outside the body. Together we are the body of Christ, and together by the exercise of our several gifts we are his witnesses in the world. That certainly seems to be the message of Ephesians 4-6.
The point is that the church is integral to the mission in the same way that Jesus was integral to the mission. Jesus was the incarnation of God in the world. All that he was and did was a picture of God's character and heart. The church as much as the individual members is the incarnation of Christ. The mission can not be carried out apart from the church. Let me say it more strongly. The mission can not be carried out apart from a healthy, Christ-like, spirit-filled church. That is the biblical model.
To prioritize the mission ahead of the church is to in some degree separate the witness of the body of Christ from the mission. No missional church proponent - except perhaps Hugh Halter in his book And - would say that, but in practice that is what happens too frequently.
Returning to my second objection to the missional church movement, that it is unwise. It is unwise to deemphasize the church because that is what every new believer becomes. It is where that new believer is taught the principles of following Jesus. It is where new believers are encouraged. It is where they are supported in prayer. It is where they see, as those outside the church can see as well, the greatest virtue of a follower of Jesus - the love we have for one another. It is where we express the absolutely foundational act of worship.
Try to be missional apart from the church. Deemphasize, fragment, or denigrate the place of the church in God's economy, neglect the church, neglect worship, love, service of one another and the mission fails. Perhaps it will not happen immediately. But it will happen, because "the mission has a church" turns God's priority on its head.
One final observation: The writings of Paul emphasize the witness of the life of Jesus in the church as the primary outreach of the church to the world. See 1 Thessalonians 1. The message of salvation rang out from them. How? By their repentance, conversion, pure worship, and hope in the return of Jesus. See 1 Corinthians 14. The church service (Ouch! A bad word to many missional proponents) was to be conducted in such a way that inquirers would clearly hear the message. You'll have to look closely for any admonition in Pauline literature to find mission prior to the church.
Please. Don't neglect the church. God loves the church, not because what the church might do but because of what the church is, the bride of his son, the people for whom Jesus died.