This is indeed a serious question. There are many references in Quran warning of hell-fire to those who believed Islam but then turned their backs and became disbelievers. These include
Apostacy, 3:72, 3:86-88, 3:90-91, 3:167, 4:137, 5:54, 9:74, 9:107, 16:106, 33:14
- do not ask for speedy doom for apostates, 46:35
-Allah alone will punish them, 73:11, 74:11
-punishment in the hereafter, 2:217, 9:74
-rejection by others, 3:87
-repentence, 3:89, 5:34, 9:5, 9:11
-on Judgement Day is too late, 40:85
-under duress, 16:106
However your question related to the death penalty for apostates (murtid). I searched the internet to find a suitable answer, and the following is one of those:
**The punishment for apostasy (riddah) is well-known in Islaamic Sharee’ah. The one who leaves Islaam will be asked to repent by the Sharee’ah judge in an Islaamic country; if he does not repent and come back to the true religion, he will be killed as a kaafir and apostate, because of the command of the Prophet (peace and blessings of Allaah be upon him): "Whoever changes his religion, kill him." (Reported by al-Bukhaari, 3017).
It is well-known in Sharee’ah that the punishments (hudood) are not carried out on minors, because they have not yet reached the age of responsibility; but in the case of those who have reached the age of responsibility, the punishment (hadd) applies, without a doubt.
The person who knows the truth and believes in it, then turns his back on it, does not deserve to live. The punishment for apostasy is prescribed for the protection of the religion and as a deterrent to anyone who is thinking of leaving Islaam. There is no doubt that such a serious crime must be met with an equally weighty punishment. If the kuffaar do not give people the freedom to cross a red light, how can we give freedom to people to leave Islaam and disbelieve in Allaah when they want to.**
And here is the other answer:
**This question may be answered by the following points:
(1) This is the ruling of Allaah and His Messenger, as the Prophet (peace and blessings of Allaah be upon him) said: "Whoever changes his religion, kill him." (reported by al-Bukhaari, al-Fath, no. 3017
(2) The one who has known the religion which Allaah revealed, entered it and practised it, then rejected it, despised it and left it, is a person who does not deserve to live on the earth of Allaah and eat from the provision of Allaah.
(3) By leaving Islaam, the apostate opens the way for everyone who wants to leave the faith, thus spreading apostasy and encouraging it.
(4) The apostate is not to be killed without warning. Even though his crime is so great, he is given a last chance, a respite of three days in which to repent. If he repents, he will be left alone; if he does not repent, then he will be killed.
(5) If the punishment for murder and espionage (also known as high treason) is death, then what should be the punishment for the one who disbelieves in the Lord of mankind and despises and rejects His religion? Is espionage or shedding blood worse than leaving the religion of the Lord of mankind and rejecting it?
(6) None of those who bleat about personal freedom and freedom of belief would put up with a neighbour’s child hitting their child or justify this as "personal freedom," so how can they justify leaving the true religion and rejecting the sharee’ah which Allaah revealed to teach mankind about His unity and bring justice and fairness to all**
Both the above answers were based on the same hadith of the Prophet. Based on the above, if some one changes his/her religion from Islam to anyother religion, including divine religions, fall under the above. That is because Islam is the most perfect of divine religions and there seems no excuse for leaving one that is perfect to go back to something, which isn't.
However, all rulings on apostates refer to those who Believed and then turned their back to Islam and became disbelievers. These may not be applicable to someone who never believed in the first place. For instance, someone born in a muslim household but never believed Islam and consequently changed his religion. As another example, similarly the first generation qadiyanis are "murtids", but the subsequent generations of qadiyanis can not be considered to be murtids as they were never on the right path at all.
The last two paragraphs above are my opinion. Feel free to disagree. Please provide any other references you can find to counter any point.
Adios!