A religion is a set of beliefs and practices, often centered upon specific supernatural and moral claims about reality, the cosmos, and human nature, and often codified as prayer, ritual, and religious law. Religion also encompasses ancestral or cultural traditions, writings, history, and mythology, as well as personal faith and mystic experience. The term “religion” refers to both the personal practices related to communal faith and to group rituals and communication stemming from shared conviction.

The world population is the total number of humans on Earth at a given time. As of May 2008, the world’s population is believed to be just over 6.7 billion. In line with population projections, this figure continues to grow at rates that were unprecedented before the 20th century, although the rate of increase has almost halved since its peak, which was reached in 1963, of 2.2 percent per year. The world’s population, on its current growth trajectory, is expected to reach nearly 9 billion by the year 2050.

The Muslims are only (roughly) 1.4 billion (20% of world population). The christians are more than 2 billion (33% of world population). The article only compared Roman catholics who are 1.3 billion to Muslims. A better comparison would be comparing Roman Catholicism to Sunni Islam (85-90% of Muslims) to see which is the most populous religious sect. Right now Sunnis are around 1.2 billion but are increasing much faster than Roman catholics, so maybe they will overtake them after 10 years or so. Muslims will certainly overtake christians numberwise (if trends continue unchanged) sometime in the third quarter of this century.

Some additional information:

World Christians 33.32% (of which Roman Catholics 16.99%, Protestants 5.78%, Orthodox 3.53%, Anglicans 1.25%), Muslims 21.01%, Hindus 13.26%, Buddhists 5.84%, Sikhs 0.35%, Jews 0.23%, Baha’is 0.12%, other religions 11.78%, non-religious 11.77%, atheists 2.32% (2007 est.)

Brazil Roman Catholic (nominal) 73.6%, Protestant 15.4%, Spiritualist 1.3%, Bantu/voodoo 0.3%, other 1.8%, unspecified 0.2%, none 7.4% (2000 census)

United States Protestant 51.3%, Roman Catholic 23.9%, Mormon 1.7%, other Christian 1.6%, Jewish 1.7%, Buddhist 0.7%, Muslim 0.6%, other or unspecified 2.5%, unaffiliated 12.1%, none 4% (2007 est.)