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As always, it's important to look at more than the pretty pictures. Pew Research has some very elegant graphs of future religious growth, but reading the fine print of their methodology tells us something more. And I see something very odd, indeed.

In the Pew reports, 'switching' is another word for conversion. Pew suggests that Christianity will have a net loss from those switching out:

From http://www.pewresearch.org/fact-tank/2015/04/23/why-muslims-are-the-worlds-fastest-growing-religious-group/

Meanwhile, religious switching, which is expected to hinder the growth of some other religious groups, is not expected to have a negative net impact on Muslims. By contrast, between 2010 and 2050, Christianity is projected to have a net loss of more than 60 million adherents worldwide through religious switching.

And the report paints a fairly stark picture; some of the graphs suggest Muslims will outnumber Christians by 2050.

However, read the fine print.

From http://www.pewresearch.org/fact-tank/2015/04/02/7-key-changes-in-the-global-religious-landscape/:

The study uses data from 198 countries and territories on fertility, age composition and life expectancy. It also looks at rates of religious switching – where data is available – and migration between countries, and puts all of these factors together to provide the best estimates for the future.

Um, where data is available? What does that mean?

From the main report:

The Pew Research Center projections attempt to incorporate patterns in religious switching in 70 countries where surveys provide information on the number of people who say they no longer belong to the religious group in which they were raised. In the projection model, all directions of switching are possible, and they may be partially offsetting.

We're only counting converts in 70 countries? Which 70 countries?

From http://www.pewforum.org/2015/04/02/appendix-a-methodology-2/#estimating-religious-switching:

The 70 countries where switching was modeled in the main scenario held 43% of the world’s population in 2010, including many countries where switching between having a religious affiliation and not having a religious affiliation is common, such as the United States, France, Australia and New Zealand. The most populous countries in which switching was not modeled are China and India.

So, we're looking at switching in 43% of the world, and not China, and India? Does this model then implicitly assume there are no converts between now and 2050 in the two most populous countries of the world?

From: http://www.pewforum.org/2015/04/02/main-factors-driving-population-growth/#the-potential-impact-of-religious-switching-in-china

Without survey data measuring patterns of switching among China’s main religious groups, it is not possible to formally model switching in China, as this report does for other countries...

As of 2010, China had an estimated 68 million Christians and 701 million unaffiliated people. Due primarily to differences in the age and sex composition of these initial populations, in the main projection scenario – which does not attempt to model religious switching – China’s Christian population is expected to grow slightly by 2050, to 71 million, while the unaffiliated population is expected to decline to 663 million.

In other words, Pew's scenario projects a world in which the major spot for Christian growth isn't growing markedly between now and 2050. This makes their total of Christians in 2050 slightly suspect.

Worse, in other charts, Pew says Christianity will have a net loss due to switching out (from Christianity to nones) - yet they don't count switching everywhere it occurs. How can we know the total net loss of Christians due to switching if not all the switches are counted?

I'll stick with data from Operation World, the World Christian Database, and Joshua Project, thank you.