Although Christianity is the largest religion in the world with an estimated number of 2.4 billion followers, the reality is that the trend may cease to continue like this by 2050 as there are reports and projections that show how Christianity might be displaced from being the largest by 2050.
Islam is often regarded as the fastest-growing religion, although it is the second most popular religion in the world. These projections are based on the reality that factors like the fertility rates of Muslims will become very high that it will cause their population to be twice its current figure by 2050.
One of the things that Islam teaches us that men are free to marry as many wives as they want to be provided if they will be fair to them all, unlike Christianity that preaches a marriage life of just a man and his wife. Also, the population of Christians will also be growing, but it might not be on the same level as that of Muslims.