We're certainly not going to go to say... the Catholic Church, and use their statistics for how many followers they have because we know the numbers they claim will be inflated. The Catholic Church has a tendency to include people of competing religions like Protestanism, Lutheranism and even Judaism... so you really can't trust the numbers they give. Wikipedia isn't very accurate either, because we all know those numbers can be easily modified by any kid with a keyboard.
Religion is always a hot topic... so when we looked around for a reliable source on what was the populations of different religions we ended up picking the National Geographic's "Geography of Religion" to come up with the following statistics (dated 2000):
1. Judeo-Christianity 33.0%
2. Islam 19.6%
3. Non-Religious/Atheist 15.1%
4. Hinduism 13.4%
5. Other 6.7%
6. Chinese-Traditional* 6.3%
7. Buddhism 5.9%
The CIA World Factbook has similar stats (dated 2007), but doesn't track Chinese-Traditional:
1. Judeo-Christianity 33.55%
2. Islam 21.01%
3. Non-Religious/Atheist 14.09%
4. Hinduism 13.26%
5. Other 12.25%
6. Buddhism 5.84%
Note: That doesn't mean Christianity grew 0.55% in 7 years, its just from a different statistical source.
What we do find interesting however is how these numbers change over time. National Geographic has been tracking world religions since before 1900:
In 1900, Believers by Percentage:
1. Judeo-Christianity 34.5%
2. Chinese-Traditional* 23.5%
3. Hinduism 12.5%
4. Islam 12.3%
5. Buddhism 7.8%
6. Other 9.2%
7. Non-Religious/Atheist 0.2%
And what you realize is two things: Islam has almost doubled in size in 100 years and nonbelievers/atheists, while technically not a religion, is the fastest growing belief structure.
If we were to use these statistics and growth rates to guess what the future holds, it suggests there will be 3 major sets of believers by 2100: Judeo-Christians, Muslims and Nonbelievers.
If you also take into account there was 115 different wars (including civil wars) between 1900 and 1999, that doesn't bode well for a peaceful 21st century. We've already had 9 wars between 2000 and 2008, and two of them could best be described as confrontations between Western Christianity and Middle Eastern Islam.
So while having accurate statistics is important the thing to remember is that they're really just a guideline and comparing those current statistics to more historical stats helps to put things in context.
That lends to the idea that the two major religions will be fighting between themselves for years to come while atheists/nonbelievers and other religions stand by the sidelines aghast at the horror of it all.
Afterall, what will the world come to when Christians and Muslims start going to war, arguing over something as silly as numbers, oil prices, the minor differences between the Bible and Quran and whether Jesus was the messiah or just a prophet?
"With or without religion, you would have good people doing good things and evil people doing evil things. But for good people to do evil things, that takes religion." - Steven Weinberg, Nobel Peace Prize Laureate.
