The point of this post, however, is not to look at whether this claim is true or false as far as the hard evidence goes, which should be fine since most Muslims making this claim don’t know the first thing about how they would go about proving it anyway (and the burden to do so rests on them). Neither is it the point of this post to challenge whether such a claim can properly be considered an argument that Islam is true even if it is the fastest growing religion, though it may be said briefly that this would no more constitute a positive argument for the truth, goodness or beauty of Islam than it would for gangrene, which also spreads quite rapidly. Rather, the point of the present post is to observe that this idea actually contradicts what is taught in Muslim sources.
To make this argument, at least two things must be born in mind:
In the first place, according to the Qur’an, before anyone was ever born, Allah extracted all people from the loins of Adam’s children and made everyone testify to Islamic monotheism:
Commenting on these ayats, Ibn Kathir said:
When thy Lord drew forth from the Children of Adam - from their loins - their descendants, and made them testify concerning themselves, (saying): "Am I not your Lord (who cherishes and sustains you)?" - They said: "Yea! We do testify!" (This), lest ye should say on the Day of Judgment: "Of this we were never mindful": Or lest ye should say: "Our fathers before us may have taken false gods, but we are (their) descendants after them: wilt Thou then destroy us because of the deeds of men who were futile?" (YA, Surah 7:172-173)
Secondly, not only did Allah extract everyone from the loins of Adam’s children before their birth and make them testify that Allah is their lord to the exclusion of all others, but everyone at the time of their actual birth “is born upon this fitrah”, as Ibn Kathir also says above. This point is confirmed by Bukhari in the following hadith:
“Allah stated that He brought the descendants of Adam out of their fathers' loins, and they testified against themselves that Allah is their Lord and King and that there is no deity worthy of worship except Him. Allah created them on this Fitrah, or way, just as He said, (So set you (O Muhammad) your face truly towards the religion, Hanifan. Allah's Fitrah with which He has created mankind. No change let there be in Khalqillah.) [30:30]….” (Tafsir Ibn Kathir)
Because Muslims believe that everyone was made to testify to the truth of Islamic monotheism and later were born Muslims, with the added idea that it is their parents who make them a Jew or a Christian, they will often tell people that Islam is their “birthright”. Similarly, when people convert to Islam, Muslims will speak of them as “reverts” rather than as converts, because on this view they are returning to the state of nature in which they were created.
Narrated Abu Huraira: Allah's Apostle said, "No child is born but has the Islamic Faith, but its parents turn it into a Jew or a Christian. It is as you help the animals give birth. Do you find among their offspring a mutilated one before you mutilate them yourself?" The people said, "O Allah's Apostle! What do you think about those (of them) who die young?" The Prophet said, "Allah knows what they would have done (were they to live)." (Bukhari, 8.77.597)
But if everyone is born a Muslim (as Muslims claim), and if the majority of people in the world are not Muslims (as Muslims are forced to admit), then this means that non-Muslims, i.e. those who comprise the majority of people in the world, actually turned from Islam some time after they were born, thus committing apostasy in some sense. It also means, and here is the punch line: According to Islamic sources, Islam is the fastest shrinking religion on the planet.