Are Muslims the fastest growing religion, and why?

Dear Justin,
This is the latest big news here…and around the world:
My question:  what do you think of this?
And, is it true that much of this growth is from “switching” or is it almost all biological growth?  What I had understood in the past was the Islam is in fact the fastest growing religion around the world—and continues to be—but not in terms of conversion growth. In terms of that, evangelical Xnity is #1.
What say ye?
It depends on whose data you are looking at. I usually use figures out of the Center for the Study of Global Christianity (publishers of the World Christian Encyclopedia, World Christian Trends, Atlas of Global Christianity, and World Christian Database). CSGC also is a co contributor to the World Religions Database which informs in part Pew Research.
Pew & CSGC differ slightly over projections. CSGC has published its differences here. In brief, although the analysis of the two (and the methodologies) are very similar, CSGC projects 3.4 billion Christians (vs Pew’s 2.9)–about 500 million more Christians.
Part of this is because CSGC looks more deeply at non-traditional forms of Christianity, such as house churches, which are mostly “under the radar.” One example is that while CSGC’s estimate of China’s current Christianity is in excess of 100 million, Pew’s is about 60 million.
The bottom line? CSGC estimates that Islam is now the fastest growing major religion (1.88% per annum), although it is not the largest religion. By 2050, the two disagree on which will be the major religion. I think projecting that far out is a bit of an academic exercise: it could be impacted by any number of factors.
As for the source of growth of Christianity: yes, it is largely demographic. Most Muslim dominated countries have birth rates much higher than the global average. For examples of this, look at the largest Muslim countries. Or, this chart, from Pew:
religionists fertility rate
Based on CSGC data, “conversions” factor in to Islamic growth largely in a couple of places (among some subsets of US populations, and in northern Nigeria), and through the force of marriage (in many places if a Christian woman marries a Muslim man she becomes Muslim–at least from a “legal” perspective if not from a “heart” perspective). By far, however, the largest portion of Muslim growth is from births to Muslim homes.