The Population Research Institute said the Western world is facing a crisis with almost every country having birth rates below the replacement rate of 2.1 children per woman, according to OneNewsNow.
The replacement fertility is the total fertility rate at which women would have only enough children to replace themselves and their partner. At 2.1 births per women, the population growth through reproduction would be about zero.
"It's because Christian and Jews are refusing to have children, refusing to get married, [and] having such low birth rates that the Muslims are going to inherit the Earth," said PRI spokesman Joseph D'Agostino to OneNewsNow. "It's not anything the Muslims are doing; it's what Christians and Jews are not doing."
However, others have refuted claims that Islam is the fastest growing religion in the world.
According to the World Christian Encyclopedia, the world growth rate of Muslims is 2.13 and for Christians it is 1.36, with the birth rate of Muslims about double that of Christians. However, Christians are out converting Muslims almost two to one (1.7:1), the Encyclopedia reports.
"There are more new Christians added to the world population than any other religion on earth every day," wrote Brother Andrew, founder of Open Doors, in a commentary in 2004. "The fact is today, that Christianity is the fastest growing religion on this most critical basis."
According to the Encyclopedia Britannica Book of the Year 2006, there are 2.13 billion Christians compared to 1.3 billion Muslims.