Estimates of the total number of Muslims range from 0.7 to 1.2 billion worldwide and 3 to 6 million in the U.S. About 20% of all people on earth follow Islam. The religion is in a period of rapid growth.

Christianity is currently the largest religion in the world. It is followed by about 33% of all people -- a percentage that has remained stable for decades. It is expected that, if current trends continue, Islam will become the most popular religion sometime in the mid-21^st century.