Worldwide
1. Christianity: 2 billion (33%)
2. Islam: 1.3 billion (22%)
3. Hinduism: 900 million (15%)
4. Secular/Nonreligious/Agnostic/Atheist: 850 million (14%)
5. Buddhism: 360 million (6%)

United States of America
1. Christianity: 159 million (76.5%)
2. Nonreligious: 27 million (13.2%)
3. Judaism: 3 million (1.3%)
4. Islam: 1 million (0.5%)
5. Buddhism: 1 million (0.5%)
6. Agnostic: 991 thousand (0.5%)
7. Athiest: 902 thousand (0.4%)

If you think the belief system you currently identify with has a solid majority on this planet, well think again! When I used to be Christian I assumed the majority of the people worldwide believed in it, when it turns out to be only 1/3. It seemed like Christianity was the majority because it does claim 76% of Americans, which I am one. However, this may point out that your faith tends to be based upon your location, rather than whether or not it's actually true. This is just more food for thought.