Ideally, I think we should measure it by asking people which religion they actually adhere to. That's why I mentioned censuses.
I agree. That would be the ideal way, but if we're not going to do it that way, we have to at least go for the most objective way we can. I'm 99% sure the U.S. census doesn't ask for religious denomination. Perhaps it should. Until it does, though, I'm not sure it's possible to get an accurate picture.
Actually, I think for the real picture of the state of a religion, we shouldn't only ask if a person considers themselves a member of a particular religion, but also whether they've attended a worship service within the past month.
Why use a month as the determining factor? Why not week? Or why not quarter? I know that in my church, someone who attended only once a month would not be considered an "active" member. At the other end of the spectrum, I know a number of Catholics who seldom attend Mass (maybe two or three times a year) but who are very adamant in defining themselves as Catholic.
I think using the denominations' own membership records is problematic, because I think most denominations do a much worse job at tracking people who leave than then do tracking people who come in, so, IMO, those records give an incomplete picture of what's actually going on.
Or... if we're going to look at membership records, I think we need to qualify them with other data. For instance, maybe we could look at how the ratio between total members to tithing members has changed over time to get a sense of the "quality" of the members in that new growth, such as it is.
Right, but that's not as easy a project as it may appear. If you were counting "tithing members" of the LDS Church, that might give you some pretty accurate figures. But many churches don't even expect their members to tithe. You've got to have a standard that kind of works across the board.
Just as a point worth noting... Roughly 800 people worldwide conver to The Church of Jesus Christ of Latter-day Saints every day of the year. Autodidact used to say that according to her sources, more people leave the LDS Church each year than join it. I never could convince her otherwise. But here's the point she refused to acknowledge: We are building a new chapel somewhere in the world every 16 hours. We aren't building chapels to sit empty. If more people were leaving than were converting, the existing chapels would accomodate the membership. The fact that this need for new buildings is real and can be measured proves her wrong. Of course not all people who convert continue to attend, but that's to be expected. Clearly, most of them do.