Peace,
1) Satan's sin is that it did not bow down to God's will because of ARROGANCE. It thought he was superior to Adam in creation. He put his reasoning before God's, although it was revealed to it directly.
2) God can forgive everthing but not refusal of His will due to arrogance. Because everthing we own is given by God. How can we be arrogant with things we owe to God.
Another thing He will not forgive is the rights of other creatures on us. Be it a human, or animal. If you did something evil to someone he will take some of your good deeds, or you will get the burden of some of his bad deeds. After all these if you are loser, that is your bad deeds is more than good deeds, you will go to Hell, until you are cleaned of your sins. Also, if you tortured an animal Allah will not forgive you. You will pay for it. (You see, it is important to be a nice person).
But if you are an ultimate loser, that is you object truth because of arrogance, then there's no hope for you. You are in hell forever. May God save all of us.
3) For Christians, Jews and other religions that issue is debated. Look at this verse:
"2:62 Those who believe (in the Qur'an), and those who follow the Jewish (scriptures), and the Christians and the Sabians,- any who believe in God and the Last Day, and work righteousness, shall have their reward with their Lord; on them shall be no fear, nor shall they grieve. "
There's another verse that says no one will punished for believing in God if there's no revealation send to them.
According to a view, these verses say:
a) Getting the message of Islam is important. Islam is not a missionary religion. Our sole mission should be being a good muslim, of whom people feel safe as Prophet pbuh says. Today many muslims distracts people, and give wrong impressions about Islam.
b) If a Christian believes in one God and that Jesus pbuh is a Prophet he perhaps will be saved. But he should not have reserves for truth itself and be open to it. For example if he says Qur'an is demonic in one breath (with motives of current relationships he holds and arrogance of course. Ppagans used to say to Muhammad pbuh "do you betray your ancestors's religion." Truth accepts no worldy reserves) then thats not good for him. Because he gets the mesage and refuses it.
But, if Qur'an rings a bell and he starts a sincere journey to truth he will inshallah be saved even if he does not die with label of Islam.
God knows better.
Unite