The act of leaving Islam is called ridda. The one who does so is called a murtad. Let's take a look at three inter-connected Islamic institutions that systematically unravel the life of a murtad.
1. Family and Friends
Muslim families are very tightly knit. For example, more than half of all British Pakistanis marry their first cousins. The story is the same with Arabs who more often than not marry within the clan. In such honor-based cultures, any un-Islamic behavior of a person brings shame upon the entire family.
An apostasy -- the ultimate dishonorable action -- would definitely cause a meltdown in a Muslim household. Most likely, the disgraced siblings of the apostate would no longer be able to marry in the family; the parents would disown "the infidel"; friends wouldn't want to be seen with such a heinous creature, and some might even report the murtad to the local imam or the state. Since nepotism is rampant in clans, a murtad would no doubt lose all his/her favorable connections with the family.
Thus, the entire inner circle of a murtad would crumble. A Muslim only has worth as long as he/she remains a Muslim.
For small "dishonorable" activities, such as a Muslim woman studying with males or working with them, dating males, refusing to wear the burqa in public, the punishment has sometimes been death. Muslim families quietly and quickly enforce these unwritten laws. The punishment by Jordan for such honor killings has been on occasion only six months. The fate of a murtad with these families would be no different.
2. Society and State
It is quite rare to find Muslims in Islamic countries who knowingly hire people of other sects or religions. For example, in Pakistan the law treats Ahmadis as non-Muslims. Ahmadis don't believe that Mohammad is the final Prophet. All their other beliefs and practices are the same as regular Muslims.
However, the Pakistani society treats them with contempt. The Ahmadis can't call their areas of worship "mosques". They're openly discriminated against in schools and jobs, only because their belief in one tiny matter is different from mainstream Muslims.
Imagine what such a society does to those who reject Islam in its entirety. No-one in the society dares associate with an apostate. The job of the apostate vanishes. If the apostate is married, then a divorce with the Muslim spouse is automatic. Any kids in the relationship go to the Muslim spouse. At every turn the apostate is asked for his/her religion. For example, applications for a national ID or a passport require everyone to classify their religion in Pakistan.
Here's how Egypt, another Islamic state, deals with murtads:
While there are, as yet, no laws against apostasy from Islam, the missionary or the convert may be convicted on other charges, for example "threatening social peace and intercommunal relations". There is, however, a Supreme Court ruling that a Moslem who apostacises is legally dead. He loses all rights and powers. He cannot withdraw funds from his accounts. Any person who kills him does not commit murder from a legal point of view because he is already legally dead. The "dead" person cannot marry or inherit. Nor is it possible for an apostate to have his identity card changed to "Christian". More than one hundred and fifty Muslims who have adopted Christianity have been detained in maximum-security prisons. They have been accused of threatening national unity.
One example from a Coptic press release, concerned the case of Dr. Abdul-Rahman who has been held in Cairo without trial for two years for breaking with Islam. He is in solitary confinement but his will has not been broken. He is undoubtedly being used as a warning to anyone else contemplating apostasy.
A recent story from Iran:
An Iranian convert to Christianity was kidnapped last week from his home in northeastern Iran and stabbed to death, his bleeding body thrown in front of his home a few hours later.
He is the fifth Protestant pastor assassinated in Iran by unidentified killers in the past 11 years. Three of the five were former Muslims, under Iranian law subject to the death penalty for having committed apostasy.
Even though certain Islamic states don't have laws on apostasy, they still go ahead with the punishment of death by hanging or beheading. Ibn Warraq:
The absence of any mention of apostasy in some penal codes of Islamic countries, of course, in no way implies that a Muslim in the country concerned is free to leave his religion. In reality, the lacunae in the penal codes are filled by Islamic law. Mahmud Mahammad Taha was hanged for apostasy in Sudan in 1985, even though the Sudanese Penal Code of 1983 did not mention such a crime.
At last, we come to the heart of the matter.
3. The Quran, Hadith and Sharia
They desire that you should disbelieve as they have disbelieved, so that you might be (all) alike; therefore take not from among them friends until they fly (their homes) in Allah's way; but if they turn back, then seize them and kill them wherever you find them, and take not from among them a friend or a helper.
Ali Sina provides the relevant context about "they." Even so, this verse can be interpreted as ordering Muslims to kill disbelievers.
They swear by Allah that they did not speak, and certainly they did speak, the word of unbelief, and disbelieved after their Islam, and they had determined upon what they have not been able to effect, and they did not find fault except because Allah and His Messenger enriched them out of His grace; therefore if they repent, it will be good for them; and if they turn back, Allah will chastise them with a painful chastisement in this world and the hereafter, and they shall not have in the land any guardian or a helper.
Muslims often carry out this delivery of chastisement.
Sahih Bukhari: Volume 9, Book 84, Number 57.
Some Zanadiqa (atheists) were brought to 'Ali and he burnt them. The news of this event, reached Ibn 'Abbas who said, "If I had been in his place, I would not have burnt them, as Allah's Apostle forbade it, saying, 'Do not punish anybody with Allah's punishment (fire).' I would have killed them according to the statement of Allah's Apostle, 'Whoever changed his Islamic religion, then kill him.'"
But remember, Islam is the Religion of PeaceTM.
Sahih Bukhari: Volume 9, Book 84, Number 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 and one of us said, 'I pray and sleep, and I hope that Allah will reward me for my sleep as well as for my prayers.'"
Ah, the sweet wet dreams of 72 raisins.
Sahih Bukhari: Volume 9, Book 84, Number 64.
No doubt I heard Allah's Apostle saying, "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, where-ever you find them, kill them, for who-ever kills them shall have reward on the Day of Resurrection."
Again, we see the talk of reward (read: heaven) for killing a murtad.
A sensible person might think that modern Muslims have outgrown these 1400-year-old writings. One might even ask, "Isn't this punishment too barbaric?" The answer from Understanding Islam is simple and illuminating:
Saying that a particular punishment is too barbaric or otherwise, is basically giving a value judgment about that punishment. If that punishment is fixed by Allah or any of His prophets, it is the belief of all Muslims that then that punishment, whether severe or soft, serves justice.
That brief answer crystallizes the pig-headed thinking of most Muslims:
If the Quran or Hadith specify a certain action, then that activity is right.
End of debate.
Let's see the prescription of Islam Online for a murtad:
If a sane person who has reached puberty voluntarily apostatizes from Islam, he deserves to be punished. In such a case, it is obligatory for the caliph (or his representative) to ask him to repent and return to Islam. If he does, it is accepted from him, but if he refuses, he is immediately killed.
Note that Sharia is not confined by geographical boundaries. If it is just to kill a murtad in Saudi Arabia, then so it is in America, Australia, Europe -- anywhere and everywhere.
The vast majority of Muslims treat murtads as traitors. In their eyes, the life of an apostate is worthless. From the closest relatives to the entire Ummah, an apostate has little to no shelter. Murtads are surrounded by a climate of suffocation in Islamic societies. That is why so many countless apostates keep their thinking to themselves. If caught, their only recourse is to profess maniacal allegiance to Islam. Otherwise, a tight noose or a sword welcomes them.
Muslims are immensely proud of their timeless religion. The Quran is respected and revered instead of being recognized for what it is -- a glorified encyclopedia of asininity. Islamic laws such as punishments for stealing, adultery, murder, rape, and apostasy remain the same across time and space. These laws are unreformable by definition.
The only hope is for Muslims to ignore such laws. Sadly, the past fourteen centuries don't offer much in that direction. Terror, both psychological and physical, is the very soul of Islam. For what would you say to "continue to believe or die"?