2. Paul's whole strategy was to plant urban churches. The greatest missionary in history, St.Paul, had a rather simple, two-fold strategy. First, he went into the largest city of the region (cf. Acts 16:9,12), and second, he planted churches in each city (cf. Titus 1:5-"appoint elders in every town").
Once Paul had done that, he could say that he had 'fully preached' the gospel in a region and that he had 'no more work' to do there (cf. Romans 15:19,23). This means Paul had two controlling assumptions: a) that the way to most permanently influence a country was through its chief cities, and b) the way to most permanently influence a city was to plant churches in it. Once he had accomplished this in a city, he moved on. He knew that the rest that needed to happen would follow.
Response: 'But,' many people say, 'that was in the beginning. Now the country (at least our country) is filled with churches. Why is church planting important now?" We also plant churches because--
B. We want to be true to THE GREAT COMMISSION. Some facts--
1. New churches best reach a) new generations, b) new residents, and c) new people groups. First (a) younger adults have always been disproportionately found in newer congregations. Long-established congregations develop traditions (such as time of worship, length of service, emotional responsiveness, sermon topics, leadership-style, emotional atmosphere, and thousands of other tiny customs and mores), which reflect the sensibilities of long-time leaders from the older generations who have the influence and money to control the church life. This does not reach younger generations.
Second, (b) new residents are almost always reached better by new congregations. In older congregations, it may require tenure of 10 years before you are allowed into places of leadership and influence, but in a new church, new residents tend to have equal power with long-time area residents.
Last, (c) new socio-cultural groups in a community are always reached better by new congregations. For example, if new white-collar commuters move into an area where the older residents were farmers, it is likely that a new church will be more receptive to the myriad of needs of the new residents, while the older churches will continue to be oriented to the original social group. And new racial groups in a community are best reached by a new church that is intentionally multi-ethnic from the start.
For example: if an all-Anglo neighborhood becomes 33% Hispanic, a new, deliberately bi-racial church will be far more likely to create 'cultural space' for newcomers than will an older church in town. Finally, brand new immigrant groups nearly always can only be reached by churches ministering in their own language. If we wait until a new group is assimilated into American culture enough to come to our church, we will wait for years without reaching out to them.
[Note: Often, a new congregation for a new people-group can be planted within the overall structure of an existing church. It may be a new Sunday service at another time, or a new network of house churches that are connected to a larger, already existing congregation. Nevertheless, though it may technically not be a new independent congregation, it serves the same function.]
In summary, new congregations empower new people and new peoples much more quickly and readily than can older churches. Thus they always have and always will reach them with greater facility than long-established bodies. This means, of course, that church planting is not only for 'frontier regions' or 'pagan' countries that we are trying to see become Christian. Christian countries will have to maintain vigorous, extensive church planting simply to stay Christian!