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Title is a bit misleading.

What are they?

“Title is a bit misleading.”
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Yup. And it plays around with words and figures. Just brought this link up earlier today in another thread.
https://www.thegospelcoalition.org/article/factchecker-are-all-christian-denominations-in-decline

Title is a bit misleading.

Gallup is far too sparse with details, but references a Pew study:

To be sure, the United States remains home to more Christians than any other country in the world, and a large majority of Americans – roughly seven-in-ten – continue to identify with some branch of the Christian faith.1

But the major new survey of more than 35,000 Americans by the Pew Research Center finds that the percentage of adults (ages 18 and older) who describe themselves as Christians has dropped by nearly eight percentage points in just seven years, from 78.4% in an equally massive Pew Research survey in 2007 to 70.6% in 2014. Over the same period, the percentage of Americans who are religiously unaffiliated – describing themselves as atheist, agnostic or “nothing in particular” – has jumped more than six points, from 16.1% to 22.8%. And the share of Americans who identify with non-Christian faiths also has inched up, rising 1.2 percentage points, from 4.7% in 2007 to 5.9% in 2014. Growth has been especially great among Muslims and Hindus, albeit from a very low base. - America’s Changing Religious Landscape

Also If current trends continue, by 2050 …

The number of Muslims will nearly equal the number of Christians around the world.
Atheists, agnostics and other people who do not affiliate with any religion – though increasing in countries such as the United States and France – will make up a declining share of the world’s total population.
The global Buddhist population will be about the same size it was in 2010, while the Hindu and Jewish populations will be larger than they are today.
In Europe, Muslims will make up 10% of the overall population.
India will retain a Hindu majority but also will have the largest Muslim population of any country in the world, surpassing Indonesia.
In the United States, Christians will decline from more than three-quarters of the population in 2010 to two-thirds in 2050, and Judaism will no longer be the largest non-Christian religion. Muslims will be more numerous in the U.S. than people who identify as Jewish on the basis of religion.
Four out of every 10 Christians in the world will live in sub-Saharan Africa. - The Future of World Religions: Population Growth Projections, 2010-2050