Apostasy from Islam. Sharia deems Ridda an illegal act, thus not permitted. Whether ridda is punishable is a question of debate and definition.
The performer of ridda is denoted murtadd (m) and murtadda (f).
It is generally assumed that the Koran only mention severe punishments for a murtadd in the afterlife. But there are passages that may well be interpreted to impose death penalty in this life, as is the case with sura 9. These aya clearly deal first with non-Muslims, but in the sense of breaking oaths, they can be said to point at murtadds as well.
Koran, sura 9
12 But if they break faith with you after their treaty, and taunt your religion, then fight the leaders of misbelief; verily, they have no faith, haply they may desist.
13 Will ye not fight a people who broke their oaths, and intended to expel the Apostle? They began with you at first, are ye afraid of them? God is more deserving that ye should fear Him! If ye be believers, 14 Kill them!
Koran, sura 8
57 Verily, the worst of beasts in God’s eyes are those who misbelieve and will not believe;
In the hadiths, on the other hand, there are numerous refereces both to severe punishment in this life, as well as references to executions or other severe reactions to murtadds in the times of the early Muslims.
Muhammad is said to have ordered the killing of anyone leaving Islam, Ali is reported to have burned murtadds, and A'isha is said to have commanded the slaying, crucifixion or banishment of murtadds.
Bukhari, Volume 9, Book 83
17God's Apostle said, The blood of a Muslim [...] cannot be shed except [...] the one who reverts from Islam and leaves the Muslims...
Bukhari, vol. 9, book 84
58... He was a Jew and became a Muslim and then reverted back to Judaism." [...] Mu'adh said, "I will not sit down till he has been killed. This is the judgment of God and His Apostle and repeated it three times. Then Abu Musa ordered that the man be killed, and he was killed ...
With all schools of Sharia, the regulations declare that a male murtadd must be killed, while a woman is to be imprisoned until she returns to the faith. While this regulation is rarely carried out in most modern Muslim communities, that does not mean it is an abandoned regulation. With some exceptions, it would be indicative of the reason why few Muslims risk their lives going public with their new non-Muslim conviction.
The concept of ridda has in modern times been expanded to include disloyalty or blasphemous acts, meaning that a Muslim can both remain Muslim and still be defined as a murtadd. Depending on the seriousness of this disloyalty or blashemy, it may be punished as severely as regular ridda.
Arabic has no specific term for Atheism, but when distinguishing between a Muslim apostate and an Atheist of non-Muslim background, the term "ilhad" is used. But "ilhad" is actually synonymous with "heresy" or "apostasy." Not distinguishing Atheism from the rejection of an original Muslim faith, but rather defining an Atheist as one who specifically has rejected God, Sharia employs ridda regulations against Atheism. This also relates to the Koran 8:57, in which the people who refuse to believe are deemed "the worst of beasts", who, according to the Koran 9:13-14, may suffer the hardest of punishments.
A second meaning of the word evolves after the death of Muhammad in 632, when there were a number of people in Arabia claiming to be prophets, who would not accept the legitimacy of Muhammad and his followers. The war fought over control of Arabia, is known as ar-Ridda, or in English, the Apostasy War. The war came to be expanded to fight tribes that never at any time had submitted to Islam. On its many fronts the Apostasy War lasted 3-4 years.
