Assalamu'alaikum
Allah forgives all sins it is true even for the one who commits shirk when he repents from doing this. BUT if he dies while he is still commiting shirk and he does not repent before he dies then he does not forgiven. This is an explanation from islam Q&A, please read..
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Praise be to Allaah.
Allaah has told us that he will forgive all sins for the one who repents to Him. He says (interpretation of the meaning):
“Say: ‘O ‘Ibaadi (My slaves) who have transgressed against themselves (by committing evil deeds and sins)! Despair not of the Mercy of Allaah, verily, Allaah forgives all sins. Truly, He is Oft‑Forgiving, Most Merciful’”
[al-Zumar 39:53]
This includes all sins, even shirk. Whoever repents, Allaah will accept his repentance.
With regard repentance from shirk in particular and the acceptance thereof, Allaah says (interpretation of the meaning):
“And those who invoke not any other ilaah (god) along with Allaah, nor kill such person as Allaah has forbidden, except for just cause, nor commit illegal sexual intercourse and whoever does this shall receive the punishment.
69. The torment will be doubled to him on the Day of Resurrection, and he will abide therein in disgrace;
70. Except those who repent and believe (in Islamic Monotheism), and do righteous deeds; for those, Allaah will change their sins into good deeds, and Allaah is Oft‑Forgiving, Most Merciful”
[al-Furqaan 25:68-70]
Allaah mentions the kufr and shirk of the Christians, then calls them to repent. He says (interpretation of the meaning):
“Surely, disbelievers are those who said: “Allaah is the third of the three (in a Trinity).” But there is no Ilaah (god) (none who has the right to be worshipped) but One Ilaah (God —Allaah). And if they cease not from what they say, verily, a painful torment will befall on the disbelievers among them.
74. Will they not turn with repentance to Allaah and ask His forgiveness? For Allaah is Oft‑Forgiving, Most Merciful”
[al-Maa'idah 5:73-74]
No matter how great the sin, the forgiveness, generosity and kindness of Allaah is greater.
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http://www.islamqa.com/en/ref/34171/shirk
