If migration of unbelievers is large the church can experience repeated bursts of revival growth (figure 1). The first revival burns out because the unbelieving pool has dropped below the threshold level. However migration increases the susceptible pool, more slowly than it was exhausted by revival, eventually taking it back over the threshold of revival. Thus a second revival occurs. the time gap between revivals depends on duration of the enthusiastic period. The simulation in figure 1 has a very short enthusiastic period.
Principles of Migration Growth
Results of the Limited Enthusiasm with Migration Model
Principles can be established for the dynamics of church growth with migration. Migration can help give:
- 1. Repeated revival growth.
- 2. Endemic revival growth.
- 3. Enhanced revival growth.
The term "revival growth" means rapid exponential growth in the numbers in church, and the number of enthusiasts. It is similar to the epidemic phase of the spread of a disease.
Thus a church on a growing housing estate can find itself growing through repeated periods of growth. This is entirely due to the growth in the estate and independent of any spiritual effects such as enthusiasts having their zeal reduced or increased. Some growth in megachurches over recent years may well be due to such a church increasing its sphere of influence through its increasing reputation or benefits offered, and thus increasing its unbelieving pool.
Figure 2 shows the extent to which the enthusiasts' numbers drop between periods of revival. The church stalls as there are insufficient enthusiasts to continue growth. The peaks in the number of enthusiasts get lower and eventually revival growth ceases. Thus the repeated revivals that occurred in Wales from 1730s to 1904 cannot all be explained by a growing population alone as the largest revivals were at the end of the period not the beginning. Thus something else must have grown as well during that period, e.g. the reproduction potential, in order to explain the size of the 1859 and 1904 revivals in terms of the previous ones.
If a community has inward migration then it is possible that some of those migrants are believers. A growing church, especially one in revival, can actively attract believers especially enthusiasts. In the previous simulation assume that 25 enthusiasts are also attracted per year by some form of migration. Figure 3 shows that the growth in the church (curve 2) is now much smoother after the initial revival than without enthusiast migration (curve 1). That is, there is no further repeated bursts of revival growth.
Because of the migration of enthusiasts their numbers no longer crash after the first peak of revival growth, but remain at a more or less constant value (figure 4, curve 2). In epidemiological terms this is the endemic case. The church sees steady growth in its numbers, but no longer growth in enthusiasts. Thus the repeated bursts of revival in history cannot be explained from attracting like-minded enthusiasts alone, there must be at least another mechanism. The eclectic church phenomenon can explain steady growth, but cannot, on its own, explain revival growth.
Clearly migration can cause a church to increase just because there are more people open to the church's influence. However because migration swells the susceptible pool it can tip a church into revival growth without any change in spiritual conditions (reproduction potential). This is due to the condition for revival growth being dependent on the number of unbelievers and the zeal of the enthusiasts alone, and not on the number of enthusiasts. Thus the principle of "migration enhanced revival" is that migration can indirectly cause revival growth by providing a new pool of susceptible unbelievers. It is likely that this principle is a factor in the rapid growth of African churches in London from the early part of the 21st century. It may become a significant factor in the future growth of practicing Islam in the UK, which appears to be in its infancy to date (2012). Migration was undoubtedly a factor in past revivals in the USA and Wales, but the speed of some of those movements, often rapid growth within weeks rather than years, rules this principle out as a major cause of growth in those revivals.