Originally Answered: Muslims: What is the penalty for apostasy in Islam? Is this put in practice in any Islam ruled countries?
Muslims ALWAYS obey their Qur'an and hadiths.
Those who turn their back on Islam are to be put to death. This is confirmed by the words and deeds of Muhammad. The only freedom of belief in Islam is the freedom to become Muslim.
Qur'an (4:89) - "They but wish that ye should reject Faith, as they do, and thus be on the same footing (as they): But take not friends from their ranks until they flee in the way of Allah (From what is forbidden). But if they turn renegades, seize them and slay them wherever ye find them"
Qur'an (9:11-12) - "But if they repent and establish worship and pay the poor-due, then are they your brethren in religion. We detail Our revelations for a people who have knowledge. And if they break their pledges after their treaty (hath been made with you) and assail your religion, then fight the heads of disbelief - Lo! they have no binding oaths - in order that they may desist." This verse is speaking of infidels (ie. "slay the infidels wherever you find them" 9:5) who obviously became Muslim to escape the sword, but the Hadith make no distinction of how a Muslim came to be a Muslim. Apostasy is always punished by death.
Other verses that seem to support the many Hadith demanding death for apostates are Qur'an verses 2:217, 9:73-74, 88:21, 5:54, and 9:66.
Advocates for killing apostates have pointed out that the supporting hadith are reliable and thus qualify as law according to verse 4:80 - "Whoso obeyeth the Messenger obeyeth Allah."
From the Hadith:
The reason why executing apostates has always been well-ensconced in Islamic law is that there is an indisputable record of Muhammad and his companions doing exactly that.
Bukhari (52:260) - "...The Prophet said, 'If somebody (a Muslim) discards his religion, kill him.' "
Bukhari (83:37) - "Allah's Apostle never killed anyone except in one of the following three situations: (1) A person who killed somebody unjustly, was killed (in Qisas,) (2) a married person who committed illegal sexual intercourse and (3) a man who fought against Allah and His Apostle and deserted Islam and became an apostate."
Bukhari (84:57) - "[In the words of] Allah's Apostle, 'Whoever changed his Islamic religion, then kill him.'"
Bukhari (89:271) - A man who embraces Islam, then reverts to Judaism is to be killed according to "the verdict of Allah and his apostle."
Bukhari (84:58) - "There was a fettered man beside Abu Muisa. Mu'adh asked, 'Who is this (man)?' Abu Muisa said, 'He was a Jew and became a Muslim and then reverted back to Judaism.' Then Abu Muisa requested Mu'adh to sit down but Mu'adh said, 'I will not sit down till he has been killed. This is the judgment of Allah and His Apostle (for such cases) and repeated it thrice.' Then Abu Musa ordered that the man be killed, and he was killed. Abu Musa added, 'Then we discussed the night prayers'"
Bukhari (84:64-65) - "Allah's Apostle: 'During the last days there will appear some young foolish people who will say the best words but their faith will not go beyond their throats (i.e. they will have no faith) and will go out from (leave) their religion as an arrow goes out of the game. So, wherever you find them, kill them, for whoever kills them shall have reward on the Day of Resurrection.'"
Abu Dawud (4346) - "Was not there a wise man among you who would stand up to him when he saw that I had withheld my hand from accepting his allegiance, and kill him?" Muhammad is chastising his companions for allowing an apostate to "repent" under duress. (The person in question was Muhammad's former scribe who left him after doubting the authenticity of divine "revelations" upon finding out that he could suggest grammatical changes. He was brought back to Muhammad after having been captured in Medina).
Reliance of the Traveller (Islamic Law) o8.1 - "When a person who has reached puberty and is sane voluntarily apostatizes from Islam, he deserves to be killed." (o8.4 affirms that there is no penalty for killing an apostate).
Apostasy is taken so seriously by Muslims that it spawned the first of many major wars within the community immediately after Muhammad's death, when several tribes wanted to leave Islam and go back to their preferred religion. They were slaughtered in such places that are now known as "Garden of Death" and "Gulley of Blood", by caliph Abu Bakr's aggressive and violent campaign to force submission (and keep the tribute payments flowing back to Mecca). Within months, even a great many Muslims were dead, including those who had memorized the Qur'an by heart.
Although it has been perfectly acceptable under Islamic law to kill Muslims who choose to embrace another religion, contemporary Muslims have realized how weak and draconian this causes Islam to appear. (A sound philosophy does not require a death threat to retain believers). As such, there is a contemporary tendency to deny fourteen centuries of Islamic teaching and even the very words of Muhammad himself - at least for Western ears.
Such defenders disingenuously quote verse 2:256 which states "Let there be no compulsion in religion, for truth stands out from error" and a fragment of verse 10:99-100, "Wouldst thou (Muhammad) compel men until they are believers?" But nearly all Muslim scholars agree that both verses were spoken by Muhammad during an earlier time in his teachings, when he did not have the power to compel others. They are abrogated by later verses, such as verse 9:29, which clearly commands Muslims to fight unbelievers until they relent and either accept Islam or a state of humiliation under Islamic rule (an obvious illustration of compulsion).
These apologists also ignore the fact that apostasy is not mere unbelief, but unbelief that follows belief, which neither of these verses address. Muhammad's recognition of the distinction is reflected in his own deeds, following his capture of Mecca, and in those of his companions following his death (the Ridda Wars). How could those closest to him have felt that there should be "no compulsion in religion" if they killed those who wanted to leave Islam?