Islam is not the fastest growing religion. It is a dying religion. Muslims, like Muhammad Saeed al Sahhaf, Saddam’s information minister, are masters of lies and this is one more lie. 

The fastest growing religion is Falun Gong. This Chinese religion was launched in 1992 and by 1998 the number of people practicing Falun Gong reached 70 million to 100 million. [Source]  

However, Muslims are polyfiloprogenitive and breed faster than others but that is the characteristic of all poor and uneducated people. The truth is that intellectual Muslims are questioning the tents of Islam and leaving it in huge numbers and only the ignorant and the terrorists are remaining steadfast.  [Source]  

Despite all these facts the claim that if a belief is popular or has many following it must be true is a logical fallacy.  

This logical fallacy is called argumentum ad numerum. This fallacy consists in asserting that the more people support a belief or a proposition, the more likely it is that that proposition is correct. This is of course a fallacy because truth can never be attained through the consensus of the majority. Prior to Copernicus and Galileo everyone agreed that the Earth is flat and the universe is geocentric. This consensus of virtually everyone did not make this belief true.

Another version of this fallacy is Argumentum ad populum. You commit the fallacy when you claim a belief is true because it is popular, like saying “thousands of people convert to Islam so it must be true”

As Bertrand Russell said: “The fact that an opinion has been widely held is no evidence whatever that it is not utterly absurd; indeed in view of the silliness of the majority of mankind, a widespread belief is more likely to be foolish than sensible.” (Source)