Data on nominal Christianity vs. people following Jesus?

Dear Justin,
Where is the best place to source data on nominal Christianity versus people following Jesus?
The whole question of nominalism, apathy, and “true believers” is a difficult one to try and figure out, for a simple reason: only God can judge the heart.
Some in America have developed surveys trying to identify those who are “born-again” or “evangelicals” (see “Measuring Evangelicalism: Consequences of Different Operationalization Strategies,” Conrad Hackett & D. Michael Lindsay, for a discussion of these methods, their differences, and their results). Unfortunately most of these surveys are questions about theological beliefs and have very little to do with actual activity: does “following Jesus” have more to do with what we believe or what we do?
There are people who would clearly say they are following Jesus–they are willing to die for him–yet they would also say they are not “American evangelicals” – they don’t like the term at all. Are they “followers of Jesus”?
Some who’ve discussed this with me have thought we could use certain activities as proxies (e.g. levels of alcoholism, drug use, or pornography, for example), but then we’re getting into a “works-based” measure, and that’s slippery ground.
In any event, there are few globally comprehensive efforts in this regard. It’s often hard enough to come up with good figures on “% Christian” in many places, let alone “% who truly believe.”
One would be Operation World and its “% evangelical” figure for every country. It’s calculated by estimating, for each denomination, the percentage that is evangelical, and then adding all of this together for the country.
Now, one could argue whether this is a true measure of “people following Jesus,” but it’s probably the closest to what most people mean by it. And if you wanted to look at a spread between “cultural Christianity” and “true Christianity,” I would theorize where the spread between Operation World’s “% Christian” and “% evangelical” is greatest is where the greatest degree of nominalism would most likely be found.
Another way to approach this question is to look at the World Christian Encyclopedia or the Atlas of Global Christianity, and the difference between “professing Christian” and “affiliated Christian” in every country.
Professing is what one says on a public opinion poll or government census; affiliated is the total number of members reported by the churches within a country.
When professing is higher than affiliated, you have people who say they are Christians but do not go to church. One can suspect nominalism here (and in fact, the WCE’s term “unaffiliated Christians” is simply professing minus affiliated, and used to be called “nominal”).
When affiliated is higher than professing, you have people who go to church but do not say they are Christians in public: these are secret believers (or “crypto-Christians,” as the Encyclopedia calls them). You can imagine that there is likely very little nominalism in this situation.
By comparing these two sources you would find that in places where unaffiliated Christianity is high, the spread between “% evangelical” and “% Christian” in Operation World is very broad. So while the two sources might not agree on specific numbers, they mostly agree in broad generalities. And apart from a few country by country surveys, that’s probably the best measure of nominalism that I know of.
Justin
Got a question? Email justin@beyond.org. I’ll answer it here. I won’t share your name.