A simple thought exercise:
The probability that the total number of baptisms in established churches (i.e., in buildings) will double (that is, 2019’s baptisms will be 2x 2018’s baptisms) is somewhere close to zero (without a significant new effort involved).
On the other hand, the probability that the total number of baptisms in as a result of the growth in movements will double from 2018 to 2019 is far higher.
The reason is obvious: traditional churches are an “industrial” form of evangelism and church planting. They can be brought to higher and higher efficiencies, but there is always a natural “ceiling” or maximum “output.” Many traditional churches have met this ceiling already. Movements, on the other hand, are a “scalable” form of evangelism, like populations. So long as each disciple can make disciples (just like each person can make a person), there is no ceiling on population growth.
While these two facts are true, I always resist projecting the future of doublings. First, it’s difficult to prove the actual rate of doublings. Baptism data can more easily be collected on annual basis from traditional churches than from movements. Anecdotally, I have seen reports from one movement that, over one week in the past month, there were more than 3,000 baptisms in one small set of cities. And I have seen most (not all) movements report significant doublings in size over the past several years. But comprehensive annual data is nearly impossible to get for a variety of reasons: lack of resources and heightened security being the more obvious ones.
Second, significant rates of growth (like annual doublings) are very difficult to maintain over time,, especially as groups get larger. Yes, populations and movements can both grow, but the reality is their annual rate of growth slows as they get larger, until new members equal the number who drop off (through death, emigration or, in the case of churches, people leaving the faith). It is almost certain that growth will continue, but rapid growth rates drop until populations are not doubling every year, or not even doublng at all.
To maintain growth rates throughout movements as a whole, it is both more efficient and effective to plant new movements rather than try to increase the speed of existing large movements. This is a strategy that many movements have adopted or are adopting.