Martin Kahler appears to have been the first theologian to say it: "Mission is the mother of all theology." I concur completely. One of the major problems of Protestant orthodoxy, especially when it becomes scholastic and academic in form, is that it is divorced from a truly missional context. Good theology grows best in the soil of good questions faced by the church in a true missionary encounter.
Several years ago Terry C. Muck asked, "Is there any other way to do theology?" He added, "A cogent argument can be made that to do theology is to do mission. An even more cogent argument can be made for the reverse: That to do mission is to do theology." Muck suggests that at every point in church history where significant debates about important theological matters occured, these debates were actually preceeded by a cultural clash that challenged some church teaching and this clash forced a more definitive theological formuation in the process. This seems self-evident to me.
I preached today on "The Conversion of the Church" from John 20:21, at Church of the King in Santa Cruz, CA. I argued that when the church is continually re-converted to faith in Christ, by engaging the world afresh, she will be regularly delivered from doing theology as an ideological and intellectual exercise. By being missional the church learns to think and confess a living faith in a truly missional context. This makes for faithful witness to both the kerygma and healthy theology. Increasingly, Catholic and Protestant missiologists are discovering this fact in new ways. Sadly, too many conservative thinkers get this process and relationship quite wrong since they have reduced faith to propositions and formulations as an end in themselves.