What is the most popular religion on Earth?

"Popular" needs some clarification, so if we talk about the most popular religion, we need to break it down a couple of different ways. I will focus first on organized religious systems and not folk religious beliefs, which will be covered at the end of this discussion. 
"Popular" religions by:
Largest number of adherents;
Fastest growing;
Broadest geographical coverage.
In terms of the largest number of adherents (hundreds of millions), Christianity tops the list with more than 2 billion followers; Islam comes in second with around 1.3 billion; Hinduism has more than 900 million; and Buddhism has more than 360 million believers.
The fastest growing religion is Islam. The Pew Research Center cites fertility rates and youth populations as being major factors in the religion's growth. If current trends continue, Muslims may equal the number of Christians by 2050.
The religion with the largest geographical coverage is Christianity, dominating North and South America, Western and Eastern Europe, Russia, sub-Saharan Africa, Australia, and the nations of Micronesia and Polynesia.
So, while Christianity has the largest number of followers spread over the greatest geographical area, Islam is the fastest growing religion and may equal Christianity's numbers in the future.
But no discussion of religion can be complete without mentioning folk religious beliefs. These are unorganized belief systems that vary widely all over the globe. These can include ancestor worship, such as Chinese traditional religion which has roughly 394 million adherents, to the worship of spirits connected to nature, animals, or auspicious places commonly known as animism.
Folk religious beliefs can exist within organized religions as well, such as Buddhism and the various sects that comprise Hinduism, Christianity, and Islam. Therefore, if we were to include folk religious beliefs as a separate category, the number of adherents would make it one of the world's largest (or popular) religions.
https://www.pbslearningmedia.org/resource/sj14-soc-religmap/world-religions-map/

https://www.pewforum.org/2015/04/02/religious-projections-2010-2050/