There were 1.6 billion Muslims in the world as of 2010 – roughly 23% of the global population – according to a Pew Research Center estimate. But while Islam is currently the world’s second-largest religion (after Christianity), it is the fastest-growing major religion.
If current demographic trends continue, the number of Muslims is expected to exceed the number of Christians by the end of this century.
There are two major factors behind the rapid projected growth of Islam, and both involve simple demographics, according to Pew Research:
Muslims have more children than members of other religious groups. Around the world, each Muslim woman has an average of 3.1 children, compared with 2.3 for all other groups combined.
Muslims are also the youngest (median age of 23 years old in 2010) of all major religious groups, seven years younger than the median age of non-Muslims. As a result, a larger share of Muslims already are, or will soon be, at the point in their lives when they begin having children. This, combined with high fertility rates, will fuel Muslim population growth.