All sins are forgivable in Islam if the person sincerely repents to Allah. The thing that cannot be forgiven if the person does not repent of it before dying is shirk or taking partners with Allah. Allah says [Allah forgiveth not that partners should be set up with Him; but He forgiveth anything else, to whom He pleaseth; to set up partners with Allah is to devise a sin most heinous indeed] (An-Nisaa’ 4:48).
Sheikh Muhammad Al-Mukhtar Ash-Shinqiti, Director of the Islamic Center of South Plains, Lubbock, Texas, states the following: Allah says in Surat Az-Zumar [Say: “O my Servants who have transgressed against their souls! despair not of the Mercy of Allah: for Allah forgives all sins: for He is Oft-Forgiving, Most Merciful] (Az-Zumar 39:53).
The Prophet (peace and blessings be upon him) also said, “Allah will accept the repentance of His servant as long as he does not breathe his last” (Ahmad).
all sins are forgivable in Islam if the person sincerely repents to Allah. Conditions of repentance are these:
1. Regretting the sin
2. Determining not to commit the same sin again
3. Returning things to their rightful owners if the sin is related to wronging people
If the sinner does not repent and dies persistent in doing the sin, then judgment is left to Allah: If He wishes He can forgive the person, and if He wishes He can punish the person.
What cannot be forgiven if the person does not repent of it before dying is shirk
or taking partners with Allah. Allah says [Allah forgiveth not that partners should be set up with Him; but He forgiveth anything else, to whom He pleaseth; to set up partners with Allah is to devise a sin most heinous indeed] (An-Nisaa’ 4:48).
In addition, rapists and murderers are not excluded from the mercy of God if they sincerely repent to Allah and fulfill the above mentioned conditions. Islam promises that the bad deeds accumulated by the sinners will be replaced by good deeds if they repent and go back to Allah. Allah says [Those who invoke not, with Allah, any other god, nor slay such life as Allah has made sacred, except for just cause, nor commit fornication, and any that does this (not only) meets punishment. (But) the Penalty on the Day of Judgment will be doubled to him, and he will dwell therein in ignominy. Unless he repents, believes and works righteous deeds, for Allah will change the evil of such persons into good, and Allah is Oft-Forgiving, Most Merciful] (Al-Furqan 25:68-70).