As of 2010, Christianity was by far the world's largest religion, with an estimated 2.2 billion adherents, nearly a third (31 percent) of all 6.9 billion people on Earth, the Pew report says. Islam was second, with 1.6 billion adherents, or 23 percent of the global population..
http://www.npr.org/sections/thetwo-way/2015/04/02/397042004/muslim-population-will-surpass-christians-this-century-pew-says
1 Islam will supplant Judaism as the second-most popular religion in the U.S.  India will displace Indonesia as the home of the world's largest Muslim population, even as the country retains its Hindu majority.
http://www.npr.org/sections/thetwo-way/2015/04/02/397042004/muslim-population-will-surpass-christians-this-century-pew-says
The world's largest religion is Christianity, making up about 33% of the world population according to a 2005 survey by the Encyclopædia Britannica. That is approximately two billion people. Other large religions include Islam (20%), Hinduism (13%), Chinese folk religion (6.3%) and Buddhism (5.9%). Indigenous religions make up 4% of the world's population, and atheism about 14%.
http://www.wisegeek.com/what-is-the-worlds-largest-religion.htm
Religiously unaffiliated people – sometimes called the “nones” – account for 16% of the world’s population, and they make up the largest “religious group” in seven countries and territories. Perhaps more remarkably, they also are the second-largest group in roughly half (48%) of the world’s nations.
http://www.pewresearch.org/fact-tank/2015/06/22/what-is-each-countrys-second-largest-religious-group/
Three-quarters of the world’s people live as part of the largest religious group in their country, while 16% are part of the second-biggest group. Muslims are the second-largest group in 30 countries.
http://www.pewresearch.org/fact-tank/2015/06/22/what-is-each-countrys-second-largest-religious-group/
Christianity has only been the world's largest religion for about 100 years. Hinduism was the first world religion and also the longest lived world religion. Buddhism has been the world's largest religion since the Mongol Empire up until the 1950s. Buddhism spread peacefully through missionary work.
http://www.wisegeek.com/what-is-the-worlds-largest-religion.htm
Islam is growing more rapidly than any other religion in the world, according to a new report by the Pew Research Center that says the religion will nearly equal Christianity by 2050 before eclipsing it around 2070, if current trends continue.
http://www.npr.org/sections/thetwo-way/2015/04/02/397042004/muslim-population-will-surpass-christians-this-century-pew-says
Indeed, in many countries, the largest group is far larger than the second-largest group. Across all countries and territories in the world, the median share of the country population in the largest religious group is 88%, whereas the median share in the second-largest group is 9%.
http://www.pewresearch.org/fact-tank/2015/06/22/what-is-each-countrys-second-largest-religious-group/
Adherents of folk religions are the largest religious group in just three countries (Macau, Taiwan and Vietnam), but they are the second-largest group in 23 nations, including China (where “nones” are the biggest group). Hindus, meanwhile, make up the second-largest group in 12 countries, perhaps most notably Pakistan.
http://www.pewresearch.org/fact-tank/2015/06/22/what-is-each-countrys-second-largest-religious-group/
1 Islam will supplant Judaism as the second-most popular religion in the U.S. 2  India will displace Indonesia as the home of the world's largest Muslim population, even as the country retains its Hindu majority.  In addition, Pew says, Four out of every 10 Christians in the world will live in sub-Saharan Africa..
http://www.npr.org/sections/thetwo-way/2015/04/02/397042004/muslim-population-will-surpass-christians-this-century-pew-says
what is the world's largest religion
Christianity is the world's largest religion.