Philip Jenkins, a scholar at Baylor University is trying to come up with an answer.
Meanwhile, he warns against unsolicited self-proclamations.
Wikipedia, the online encyclopaedia which can be edited by users, includes the article “Claims to be the fastest-growing religion”.
It contains a host of statements relating to Islam, Buddhism, Christianity, Wicca, Deism, Falun Gong, Scientology and other religions.
Each individual who wrote the statements claims no other religion is growing as fast as theirs.
Obtaining accurate figures is difficult if not impossible, since probing into other's souls is inconceivable.
According to Jenkins, however, it is possible to at least try to offer a rough outline.
At first glance, the answer appears pretty obvious, with Christianity coming out on top.
In the 20th century, the number of Christians in the world grew exponentially, particularly in the southern hemisphere.
The World Christian Database, which is considered a reliable source, reveals that since 1900, the number of Christians in Africa has grown by 4930% and in Latin America by 877%. Catholics in Africa during that period went from 1, 9 million to 130 million: a 6700% increase.
But Jenkins says that before the prize is awarded, we need to take a look at what was happening in the Islamic faith at the time. If the figures are seen as a percentage of the world population, the picture changes quite radically.
Both Christianity and Islam enjoyed a strong growth during the last century.
But Muslims have altered their figures more than Christians (who have grown exponentially as a population) have.
But despite this growth, their quota in relation to the world’s population remained the same.
In 1900 around a third of the Earth’s population was Christian and the same is true now.
And things are not likely to change according to Jenkins: in 2050 Christians will make up approximately 30% of the world’s population.
Then there are Islam’s figures: In 1900 the Prophet’s followers numbered approximately 200-220 million that is about 12-13% of the world’s inhabitants.
Now, the percentage of the Earth’s Muslims has reached 22, 5%. In 1900, there were 2, 8 Christians for every Muslim.
Today, the ration of Christians to Muslims is 1, 5:1. In 2050 this ratio will be 1, 3:1. Jenkins affirms that the gap is growing rapidly in historical terms.
It is also true, however, that while Christians can point to Africa - a continent which has historically been non Christian - as a land of development, from a statistical point of view, the growth of Islam is mainly taking place in the part of the world that has always been “theirs”.
In 1900 there were approximately 9 million Muslims living in Egypt.
Today there are 75 million.
Iran has gone from 10 million to 65 million.
The Muslim population in Indonesia amounted to approximately 34 million, compared to the 190 million that lie there today.
The explanation for this is Europe’s slow population growth during the last century, compared to the rest of the world.
So the answer to the question “Which religion is growing fastest?” is Islam; and/or Christianity, but outside Europe.