There are three issues that make counting members of a church difficult:
1 The view that somehow the collection of statistics is sinful. The legitimacy of measurement has often been raised from a theological or spiritual stand point. The argument is often that it lends to attitudes that are pessimistic or lack faith. Another argument occasionally rendered is the case of David in 2 Samuel 24, who sinned by numbering the people. I usually point out that there are plenty of “right censuses” in the Bible, and David’s problem was his heart motive. Simply put, there are a huge number of numbers in the Bible. There are censuses, measurements, symbolic numbers, etc. Moreover, we have the example of many people in the Bible who were directly ordered to survey or scout out the land–which is exactly what a missionary survey accomplishes.
2 Differences over which religion to count a person with. For example, a person could be an affiliated Roman Catholic but practice spiritistic or animistic habits. Or, he might be a Muslim but practice shamanism. The greatest example of this is in the “bleed-over” between the various Asian religions, Chinese folk religions, and Buddhism. It is difficult to measure these, but religious demography tries to get at it with the idea of “doubly religious.”
3 Differences over who “counts” as a member. Must one be baptized? Must one be an adult? Children, according to Barrett’s statistics, make up slightly over a third of all professing and practicing Christians. Are they to be counted in affiliation numbers? Many churches would say no.
Affiliated Christians are counted by gathering the membership numbers of every denomination within a country. Usually this is done through obtaining yearbooks, etc. from the churches. One can sometimes even find these at used bookstores! But more commonly, they are gathered through direct correspondence with the larger denominations.
In the World Christian Encyclopedia, two numbers are provided: adults and affiliated. The latter encompasses children, and sometimes has to be estimated on the basis of demographic information about the country itself. It’s not precise, but it’s accurate to within the correct order of magnitude.
In order to provide for comparisons, care must be taken to have statistics fixed at a similar date in time. We must not forget the enormous impact of population growth on the church. If you have statistics for one church in 1965 and for a different church in 1972, you’ll have to do some estimation using established methodologies to get them both to ‘1975’. Careful assessment of printed literature must be made: I have books on the church in America, for example, that uses the exact same number for a given church for 1925, 1930, 1935, right on up to 1965. It’s amazing that for 40-odd years, the million-plus members never died, moved, and had no new births!
In some countries, of course, getting exact numbers out of books is impossible. That’s where on-the-spot estimates are going to come into play.
Affiliation also brings in another issue, which we’ll deal with later: practicing vs. non-practicing. A person can be a “member” of the church on the rolls - baptized there, comes at least once a year - but not be a practicing member of the church.