"Christianity has become massively accepted as the religion of developing countries in the so-called Third World," said the 1,700-page World Christian Encyclopedia, prepared by the Global Evangelization Movement research center in Richmond, Va. The report details statistical information about religious groups around the world.
The report pegged Christianity as the "most extensive and universal religion in history," pointing out that Christians constituted the largest religious group worldwide at both the start and end of the 20th century.
In 1900, Christianity boasted some 555 million adherents (32.2 percent of the world's population then). In the year 2000, the religion claimed 31 percent of the world's population with more than 1.9 billion followers.
The report noted that 386 million Christians identify themselves as members of "independent" churches distinct from Catholics, Protestants, Anglicans and Orthodox Christians. In the United States, 192 million people say they are Christians.
Islam followed Christianity as the world's second most-populous religion, with 1.2 billion followers, some 19.6 percent of the world's population. At the dawn of the century, Islam claimed 200 million followers, about 12.3 percent of the world's population.
In the past three decades alone, the number of Muslims in the United States more than quadrupled to 4.1 million.
Also increasing since 1900 is the number of people who identify themselves as non-religious, the report found. Though 3 million said they were non-religious in the year 1900, last year that number stood at 768 million -- 12.7 percent of the world's population.
Behind Islam and Christianity came Hinduism with 811 million followers worldwide. Buddhism has 360 million followers around the globe, while Sikhism and Judaism boast 23 million and 14 million followers, respectively.
Acknowledging the lack of an exact consensus regarding the statistics for non-Christian religions in the United States, the report found that Judaism has 5.6 million followers in the United States. That number drops to 2.4 million for Buddhists, and 1 million for Hindus.
Worldwide, the biggest decline in followers was found among traditional belief systems -- particularly in China and Africa.