Islam has experienced many an apostatic upsurge during the course of its history. The most powerful of them was the one that manifested itself among the Arab tribes soon after the Prophet’s death. By this we mean the mightily rebellious movement that was nipped in the bud by Caliph Abu Bakr through his matchless courage and strength of will. The then, again, the second great onslaught of apostasy within Islam was the widespread swing towards Christianity with the expulsion of the Muslims from Spain. This stole into other countries too that were then under the demonation of the Western Christian powers, and was actively supported and encouraged by Christian missionaries. Apart from these well-known episodes, there are also some stray instances such as that of a few faint-hearted Muslims going over to some other faith in India. But such cases have been very rare, and the fact is that with the exception of the large-scale conversion of Muslims to Christianity in Spain, if it can be described as an apostatic movement, the general opinion of Islamic historians is that the Muslim millat has never seriously had to encounter a general threat of apostasy.
In the two Sahihs, it is reported that the Messenger of Allah (pbuh) said,
“By Allah, he does not believe. By Allah, he does not believe. By Allah, he does not believe.” Someone asked, “Whom, O Messenger of Allah? And he said, “He whose neighbor is not safe from his evil conduct.” (Bukhari)
In another narration, it is reported that,
“The one whose neighbor is not safe from his evil conduct will not enter Paradise.” (Muslim)
The Messenger of Allah (pbuh) was asked about the greatest of sins and he said,