There a few things that the Koran says can damn a believer to Hell. First, the Koran says that if a Muslim considers another religion and turns away from the Islamic faith, they will go to hell. This is called the sin of Shirk.

And whoever acts hostilely to the Messenger after that guidance has become manifest to him, and follows other than the way of the believers, We will turn him to that to which he has (himself) turned and make him enter hell; and it is an evil resort. Surah 4:115

Any one who, after accepting faith in Allah, utters Unbelief,- except under compulsion, his heart remaining firm in Faith - but such as open their breast to Unbelief, on them is Wrath from Allah, and theirs will be a dreadful Penalty. Surah 16:106

This would obviously create a fear in a Muslim of considering another faith like Christianity.

Another immediate damnation is killing another Muslim.

Whoso slayeth a believer of set purpose, his reward is hell for ever. Allah is wroth against him and He hath cursed him and prepared for him an awful doom. Surah 4:93

Also, deserting the army is cause for a sentence to hell.

whoever shall turn his back to them on that day-- unless he turn aside for the sake of fighting or withdraws to a company-- then he, indeed, becomes deserving of Allah’s wrath, and his abode is hell; and an evil destination shall it be. Surah 8:16

A Muslim cannot deny Allah or feel he cannot make us new creations (resurrect us).

And if you would wonder, then wondrous is their saying: What! when we are dust, shall we then certainly be in a new creation? These are they who disbelieve in their Lord, and these have chains on their necks, and they are the inmates of the fire; in it they shall abide. Surah 13:5

A Muslim, according to the Koran, must be careful or their actions and beliefs could doom them to the “fire.” They could make a profession of faith but do one wrong thing and forever seal their destination in hell. Because of these things, a Muslim could lose his eternal status.

According to the Bible, a Christian cannot lose his salvation, once God grants him it.

For it is by grace you have been saved, through faith—and this not from yourselves, it is the gift of God—not by works, so that no one can boast. Ephesians 2:8-9

If salvation is a gift, given to us not by our works, how can our works undo that gift? Besides, if it is a gift, who takes back a gift?

For I am convinced that neither death nor life, neither angels nor demons, neither the present nor the future, nor any powers, neither height nor depth, nor anything else in all creation, will be able to separate us from the love of God that is in Christ Jesus our Lord. Romans 8:38-39

This verse makes a case for eternal security, which means believers can never lose their eternal destination of heaven once they profess a belief in Jesus Christ. This verse applies to those who truly believe and receive God’s love. This does not apply to those who have never accepted God’s invitation for salvation. They will be cut off from God’s love for eternity since they have turned their back on him from the beginning.

However, once a believer accepts the invitation, they are eternally secure. They can never lose it, no matter what the world throws at them.