3 Kinds of Growth, and when each is important

One of the diagrams I am always careful to go over when teaching Perspectives Lesson 9 is the Global Religious Dynamics diagram. You can see it online here.
This diagram highlights two forms of growth globally: demographics (the newly born minus those who die) and conversion (those who become Christians minus those who defect).
When you look at a specific place (e.g. a country, province, city, whatever) you can add a third form of growth, immigration: those who move in minus those who move out.
Church structures generally optimize for one form of growth, be it demographic or conversion (with care for immigrants often a kind of middle point). Both are important. If Christianity in an area has conversion growth, but loses demographic growth, its share of the population will steadily erode. If it has demographic growth but no conversion growth, its share of the population will likely remain steady but it will not increase; thus, people with no access to the Gospel will slowly drip away into a Christless eternity (this being the general case today–the % of the world that is Christian has remained a steady 33% for over a century, with little change).
Some forms of church growth are optimized for building the Christian family: essentially, helping children of Christians choose Christ themselves, grow in maturity, marry believers, and in turn raise believing children. As already noted, this function is not to be downplayed. Right now globally there is a very high defection rate (15 million converts p.a. vs. 12 million defectors); if the defection “back door” were “closed,” we would see a significant jump in % Christian worldwide. (15 million per annum would add 33% to the total growth rate.)
Other forms of church growth are optimized for conversion and discipleship of new believers, and the rapid expansion of the church. This is especially necessary in view of the fact that the total number of non-Christians is twice Christians.
What I’m thinking about today, however, is this: we need both kinds of growth, but sometimes we need one kind more than others. For example, in low-% Christian countries, we need to focus on conversion and rapid expansion. But when the church reaches a certain size – say, more than 60% of the population? – demographic growth becomes very important for maintaining stability over the long run. Births become the primary engine of growth for the future of the church in that place, and the church should begin investing in conversion growth in distant places.
A church that optimizes on demographic growth (seeing most of its baptisms, for example, be the children of believers) when the % Christian in an area is low, is a poor strategy (at least in terms of the fate of the lost). But a church that optimizes on conversion growth when % Christianity is very high may be risking the loss of its children (if little emphasis is given to internal discipleship).
Disclaimer: I’m not saying churches should do one to the exclusion of all else, but in my experience the reality is churches will generally do one kind of ministry really well. Megachurches may be an exception to this, as they have more resources.