From Tim Keller:
All in all, church planting helps an existing church the best when the new congregation is voluntarily ‘birthed’ by an older ‘mother’ congregation. Often the excitement and new leaders and new ministries and additional members and income ‘washes back’ into the mother church in various ways and strengthens and renews it. Though there is some pain in seeing good friends and some leaders go away to form a new church, the mother church usually experiences a surge of high self-esteem and an influx of new enthusiastic leaders and members.
However, a new church in the community usually confronts churches with a major issue–the issue of ‘kingdom-mindedness’. New churches, as we have seen, draw most of their new members (up to 80%) from the ranks of the unchurched, but they will always attract some people out of existing churches. That is inevitable. At this point, the existing churches, in a sense, have a question posed to them: “Are we going to rejoice in the 80%–the new people that the kingdom has gained through this new church, or are we going to bemoan and resent the three families we lost to it?” In other words, our attitude to new church development is a test of whether our mindset is geared to our own institutional turf, or to the overall health and prosperity of the kingdom of God in the city.
Any church that is more upset by their own small losses rather than the kingdoms large gains is betraying its narrow interests. Yet, as we have seen, the benefits of new church planting to older congregations is very great, even if that may not be obvious initially.