Religious Statistics:

(all numbers are approximate)

World Population: 6.8 billion 

Christianity: 2.3 billion adherents; 30% of world population

Islam: 2.2 billion adherents; 29% of world population

Hinduism: 1 billion adherents; 15% of world population

Non-Religious (agnostic, atheist, secular humanist): 750 million adherents; 11% of world population*

Buddhism (includes mixes with Taoism, Confucianism, Shinto, Animism, and other folk beliefs; Buddhism alone would be closer to 400 million; secular Buddhism can include up to 1.5 billion): 680 million adherents; 9% of world population

Sikhism: 30 million adherents

Judaism (including secular Jews): 14.5 million adherents

Baha’i: 7 million adherents

Jainism: 4.2 million adherents

*As of 2007, The United States does not figure in the top twenty countries with the greatest proportion of irreligious people.  The top five are: Japan (averaged 76%), Sweden (averaged 65.5%), Denmark (averaged 61.5%), Macau (averaged 60.9%), and Czech Republic (averaged 57.5%).

An older (2005), but nicely detailed chart:

An Approximation of Percentages ca. 2011:

A chart adding the break downs in Canada and the United States:

A Chart also dating from 2011:

Religiosity, 2006-8:

Prevailing religions, per country: