However we should not misunderstand this into thinking that those who commit shirk are totally doomed. If a person commits shirk and then wishes to make repentance, Allah accepts his repentance and wipes the slate clean. But if the person does not make repentance during his lifetime, then Allah will not forgive this person on the day of judgment. It must also be remembered that Allah through his justice, has not discriminated in the punishment of this crime. If a Muslim commits shirk during his lifetime, and does not make repentance, he too will face the eternal fury of the Hellfire.
Some people say that eternal Hellfire is unjust.
But indeed Allah is the Most Just. While some will go to Hell forever for committing shirk, there will be others who will go to Paradise forever for upholding Tawheed. But we know that those who claim this, would never themselves refuse an everlasting reward such as Paradise, thus showing the weak human understanding of this argument. By committing shirk we should not fool ourselves into thinking that we harm Allah in anyway. Rather we harm ourselves with the threat of Allah's punishment.