GodlyVerses

Indeed, those who have believed then disbelieved, then believed, then disbelieved, and then increased in disbelief - never will Allah forgive them, nor will He guide them to a way. - Quran An-Nisa(The Women) 4:137 (islam)

They wish you would disbelieve as they disbelieved so you would be alike. So do not take from among them allies until they emigrate for the cause of Allah. But if they turn away, then seize them and kill them wherever you find them and take not from among them any ally or helper. - Quran An-Nisa(The Women) 4:89 (islam)

Indeed, the penalty for those who wage war against Allah and His Messenger and strive upon earth [to cause] corruption is none but that they be killed or crucified or that their hands and feet be cut off from opposite sides or that they be exiled from the land. That is for them a disgrace in this world; and for them in the Hereafter is a great punishment, - Quran Al-Ma'idah(The table served) 5:33 (islam)

And whoever opposes the Messenger after guidance has become clear to him and follows other than the way of the believers - We will give him what he has taken and drive him into Hell, and evil it is as a destination. - Quran An-Nisa(The Women) 4:115 (islam)

They went out from us, but they were not of us; for if they had been of us, they would no doubt have continued with us: but they went out, that they might be made manifest that they were not all of us. - Bible 1 John 2:19 (bible)

Whoever disbelieves in Allah after his belief... except for one who is forced [to renounce his religion] while his heart is secure in faith. But those who [willingly] open their breasts to disbelief, upon them is wrath from Allah, and for them is a great punishment; - Quran An-Nahl(The Bee) 16:106 (islam)

And whoever turns away from My remembrance - indeed, he will have a depressed life, and We will gather him on the Day of Resurrection blind." - Quran Taha(Taha) 20:124 (islam)

They ask you about the sacred month - about fighting therein. Say, "Fighting therein is great [sin], but averting [people] from the way of Allah and disbelief in Him and [preventing access to] al-Masjid al-Haram and the expulsion of its people therefrom are greater [evil] in the sight of Allah. And fitnah is greater than killing." And they will continue to fight you until they turn you back from your religion if they are able. And whoever of you reverts from his religion [to disbelief] and dies while he is a disbeliever - for those, their deeds have become worthless in this world and the Hereafter, and those are the companions of the Fire, they will abide therein eternally. - Quran Al-Baqarah(The Cow) 2:217 (islam)

And whoever desires other than Islam as religion - never will it be accepted from him, and he, in the Hereafter, will be among the losers. - Quran Ali 'Imran(The Family of Imran) 3:85 (islam)

Indeed, those who reject the message after their belief and then increase in disbelief - never will their [claimed] repentance be accepted, and they are the ones astray. - Quran Ali 'Imran(The Family of Imran) 3:90 (islam)

On the Day [some] faces will turn white and [some] faces will turn black. As for those whose faces turn black, [to them it will be said], "Did you disbelieve after your belief? Then taste the punishment for what you used to reject." - Quran Ali 'Imran(The Family of Imran) 3:106 (islam)

They swear by Allah that they did not say [anything against the Prophet] while they had said the word of disbelief and disbelieved after their [pretense of] Islam and planned that which they were not to attain. And they were not resentful except [for the fact] that Allah and His Messenger had enriched them of His bounty. So if they repent, it is better for them; but if they turn away, Allah will punish them with a painful punishment in this world and the Hereafter. And there will not be for them on earth any protector or helper. - Quran At-Tawbah(The Repentance) 9:74 (islam)

Has there not come to you the news of those who disbelieved before? So they tasted the bad consequence of their affair, and they will have a painful punishment. - Quran At-Taghabun(Mutual Loss and Gain) 64:5 (islam)

Chapter: The signs of Prophethood in Islam
Narrated Anas:
There was a Christian who embraced Islam and read Surat-al-Baqara and Al-`Imran, and he used to write (the revelations) for the Prophet. Later on he returned to Christianity again and he used to say: "Muhammad knows nothing but what I have written for him." Then Allah caused him to die, and the people buried him, but in the morning they saw that the earth had thrown his body out. They said, "This is the act of Muhammad and his companions. They dug the grave of our companion and took his body out of it because he had run away from them." They again dug the grave deeply for him, but in the morning they again saw that the earth had thrown his body out. They said, "This is an act of Muhammad and his companions. They dug the grave of our companion and threw his body outside it, for he had run away from them." They dug the grave for him as deep as they could, but in the morning they again saw that the earth had thrown his body out. So they believed that what had befallen him was not done by human beings and had to leave him thrown (on the ground). - Hadith Al Bukhari 3617 (islam)

No Muslim becomes an unbeliever (kafir) through wrong actions. - Fiqh Rissala 1.8d (islam)