Are there any unforgivable sins or mistakes in Islam? By "unforgivable" I mean sins which leave a mark on the person forever and the person cannot accommodate for it.
If so, what are they?
“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.” [Sûrah al-Zumar: 53]
“And those who invoke not any other ilah (god) along with Allah, nor kill such a person as Allah has forbidden, except for just cause, nor commit illegal sexual intercourse and whoever does this shall receive the punishment.The torment will be doubled for him on the Day of Resurrection, and he will abide therein in disgrace;
Except those who repent and believe (in Islamic Monotheism), and do righteous deeds; for those, Allah will change their sins into good deeds, and Allah is Oft‑Forgiving, Most Merciful.”
Based on these verses, no sin is too great for Allah to forgive for the one who repents. This Hadith also clarifies it.
Al-Tirmidhi narrated that Anas ibn Malik (may Allah be pleased with him) said: I heard the Messenger of Allah (peace and blessings of Allah be upon him) say: “Allah, may He be blessed and exalted, said: ‘O son of Adam, so long as you call upon Me and ask of Me, I shall forgive you for what you have done, and I shall not mind. O son of Adam, were your sins to reach the clouds of the sky and were you then to ask forgiveness of Me, I would forgive you and I would not mind.’” [Saheeh al-Tirmidhi]
So Allah will forgive your sins if your repent sincerely and do good for taking away your sins. However, there is one sin people think is not forgiven, Shirk. The verses above show that ALL sins are forgiven if one repent, but if you commit Major Shirk( if one dies while still commiting shirk), then it won't be forgiven.
Allaah Ta’ala says :
“Allah does not forgive (the sin of) joining other gods with Him; but He forgives whom He pleases other sins than this.”[4:116].
The meaning of the verse is that whoever dies while a Mushrik (polytheist) Allah will not forgive him and he will surely be punished for this sin, i.e. he will remain in Hell-fire forever. As for the person who repents from Shirk whether small Shirk or major, Allah forgives his previous Shirk and accept his repentance. Allah Ta’ala says :
“Say to those who have disbelieved, if they cease (from disbelief) their past will be forgiven. ” [8: 38].
There is no sin that is unforgivable. But there may be some conditions. As in some sins one just have to ask Allah Subhana wa Ta'ala for forgiveness and in some cases one will have to compensate for the action. As if one hurts someone else, he will have to ask for forgiveness to that person too. If one has defrauded someone, he will have to compensate. With these conditions, every sin is forgivable.
Allah subhana wa ta'ala is Merciful.
All sins can be forgiven except for one. It's called shirk. Every sin you have made can be prayed away and erased by doing many things except this. It's because this sin is believing that the lord is anyone other than Allah and believing in things other than what Allah says. Other than that, like I said before, the other sins are forgivable but never ever this one and it will permanently doom you to hell.