PEW Study has published a paper where it states that with approximately 15% of the World Population, it is the third largest Religion.
It has to be noted that while arriving at this conclusion, the innumerable sects of Christianity,like Protestants,Catholics,Greek Orthodox, and many more have been clubbed under Christianity as a whole for calculation.

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In the case of Muslims, Shia and Sunni sects and various other Group like Wahabi and the like have been taken together to arrive at Muslim population.
Hinduism is the root for Buddhism, Jainism and Sikhism.
Buddhism is more akin to Non Dualism, the difference is that while Buddhism ends up with Nireeswara vada or nihilism Advaita or Non Dualism declares the Reality to be non- one .
There are more finer points on the proximity of Hinduism and Hinduism so much so Buddha is consideerd as an Avatar of Lord Vishnu and Adi Shankaracharya is even called a ‘Pseudo Buddhist!’
One can find fundamental unity in Principles between Jainism and Hinduism.
And it is no secret the founders of these two Religions were Hindus and they have kept the essentials of Hinduism in their thoughts.
Separate blog follows on the subject.
Sikhism , no body needs to remind, is closer to Hinduism than Jainism and Buddhism.
These three Religions are more close to Hinduism than Christianity and Islam to their sects.
So clubbing these three Buddhism, Jainism and Sikhism to Hinduism is equally logical as the clubbing of the sects of Islam and Christianity in one lot.
On these lines,the population figures should be,
But ‘Indian-Hindu root religion’ is still the largest religion of world if you consider Hindus (15%) + Buddhist (7%) + Other Religions (nearly 1% most of them are Sikhs, Jains etc) + Religiously Unaffiliated (16% most of them are in China and hide their practicing Buddhist culture due a oppressive Communist Govt & others actually follow the offshoot of Hindu faiths)……… SO THERE IS AT LEAST 35% INDIA ROOTS RELIGIONISTS IN THIS WORLD LARGER THAN 32% CHRISTIANS AND 23% MUSLIMS SO FAR.
(hinduexistence.org)
By this logic Hinduism is the largest Religion in the World.
With such a population in the world, Hindus have one country to call as Hindu”Nepal!
If the 32% stake holders Christian has about 30 declared Christian Countries (157 Christian-majority nations) in this world….
The 23% stake holders Muslims have 57 declared Muslim Countries….
The 7% stake holders Buddhists have 8 Buddhist Countries….
The 0.2% stake holders Jews have 1 Jews Country…
http://hinduexistence.org/category/hindus-under-christian-aggression/
http://www.pewforum.org/2012/12/18/global-religious-landscape-hindu/

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