Allah is All-Merciful and Oft-Forgiving. Indeed, He may forgive all sins except for one sin: shirk.
In Islam, shirk is the sin of idolatry or polytheism. i.e. the worship of anyone or anything other than the singular God, or more literally associating partners with Him.
If a person dies in a state of polytheism, every hope for his or her salvation is surely dashed.
The Prophet (peace be upon him) was asked: what is the greater sin , he said: To ascribe partners to Allah even though he created you. (Al-Bukhari)
However, Almighty Allah may forgive every sin, without exception, from a sincere penitent.