The unbelievers among the People of the Book and the pagans shall burn for ever in the fire of Hell. They are the vilest of all creatures. (Koran 98:1-8)People of the Book refers to any holy scriptures revealed to man from God before his final revelation to the prophet Muhammad. It is especially used for Christians and Jews. The moral of this verse is that you Christians are going to "burn for ever in the fire of Hell" for believing in the divinity of Jesus and the integrity of the Holy Bible.
If this does at all not bother you or give you pause, then maybe you see why when you say we atheists will burn in hell forever neither bothers us. The only reason you think it should cause us concern is because you believe it to be true. The reason it doesn't cause us concern is because we do not believe the Bible is the "Word of God." Similarly, Muslims believe the above to be true about you; the reason it shouldn't give you pause is because you don't believe that the Koran is the "Word of God."