The Trinity is, by nature, "sending." The Father sends the Son into the world to save it, and the Father and the Son send the Spirit into the world. And now, said [David] Bosch, the Spirit is sending the church. In short, God does not merely send the church in mission. God already is in mission, and the church must join him. This also means, then, that the church does not simply have a missions department; it should wholly exist to be a mission. [Tim Keller, Center Church: Doing Balanced, Gospel-Centered Ministry in Your City (Grand Rapids, MI: Zondervan, 2012), 251]
The biblical God is by nature a sending God, a missionary God. The Father sens the Son; the Son sens the Spirit and his disciples into the world. Therefore the whole church is in mission; every Christian is in mission. ... A missional church, then, is one that trains and encourages its people to be in mission as individuals and as a body. (Ibid., 259)
The idea of being "missional" comes from within Liberal Christianity, and for some strange reason has been adopted by parts of Evangelicalism. Keller in his book acknowledges the problems with some versions of being "missional," while yet adopting the general thrust of the term.
Our first problem with being "missional" is that it flattens the distinction between Creator and creature. One cannot argue from the inner workings of the imminent Trinity and see the sending of the Spirit as similar to the sending of the Church. This is of course not to mention that it is God who sends the Church— Father, Son and Spirit, not just the Spirit. The whole idea of a chain of being sent erases the Creator-creature distinction, as if the sending of the Church is the next step after the Father sent the Son and the Son sent the Spirit.
The next major problem is that it portrays God as a "missionary God." If by that, it is meant that God is portrayed as seeking out the lost, then sure. But if by that, we portray God as being a missionary like us, then such is extremely problematic. God does not come and seek the lost like how missionaries desire to gain new believers for the kingdom. Rather, God already has a plan and His plan works for the salvation of many, all whom He has predestined to salvation. God's plan is implemented through His Church, and while the Church is a "mission," the Church is not just a mission. Rather, God is creating a people, of which missions is merely one aspect of what the church should be doing. The main motif describing the Church is a people and a witness (μαρτυς; martys, from which we get the word "martyr") for God. While the Church do missions in the sense of seeking out the lost, missions is not the defining nature of the Church.
According to Scripture therefore, God cannot be said to be a "missionary." God also has a plan and implements it through the Church, not apart from her. As such, the Church is not called to be "missional," in the sense in which it joins "God's mission," whatever that is. Rather, the Church is to be a witness, and missions are one aspect of her work of witness.