For most Christmas season means parties, gifts and some quality time with friends and family. The season is most important for Christians, who believe Jesus Christ was born on Christmas Day. Christianity is the largest religion in the world and according to the Pew Research Center, is likely to remain so for the next four decades.

As of 2010, the latest year for which figures are available, there are an estimated 2.2 billion Christians around the world. They represent a third of the world's population and are geographically widespread with the Americas boasting of the largest number and the highest proportion of Christians. The number of Christians worldwide has risen in the last century from an estimated 600 million in 1910 to over 2 million in 2010.

Between 2010 and 2050, the world’s total population is expected to rise by about 35 percent. While the number of Christians worldwide is expected to rise by 35 percent, the number of Muslims around the world is expected to shoot up by 73 percent. As a result, by 2050, the number of Muslims will nearly equal to the number of Christians in the world for the first time.

In 2010, Christians made up over three-quarters of the total U.S. population. However, by 2050, their numbers are likely to dwindle to about two-thirds. Islam is also set to beat Judaism as the largest non-Christian religion in the country. The number of Christians in the U.S. is projected to decline from 78 percent in 2010 to 66 percent in 2050 while the number of people following other religions is expected to nearly double.

Meanwhile, the number of people who identify themselves as atheists, agnostics and who do not affiliate with any religion is expected to decline worldwide. However, their numbers are expected to rise in the U.S. and France.

In the U.S., the number of people who are not unaffiliated to any religion is projected to grow from an estimated 16 percent of the total population in 2010 to 26 percent in 2050.