Reconciling between the hadeeth which says that whoever says Laa ilaaha ill-Allaah will enter Paradise and the idea that the mushrikeen will abide in Hell forever
Question

                How can we reconcile between the words of the Prophet (peace and blessings of Allaah be upon him), “There is no slave who says Laa ilaaha ill-Allaah then dies believing in that but he will enter Paradise,” and the idea that the mushrikeen and hypocrites will abide in Hell forever even though they said Laa ilaaha ill-Allaah?.
Praise be to Allah.
    Shaykh Muhammad ibn ‘Uthaymeen (may Allaah have mercy on him) 
    said: 

    The hadeeth indicates that the one who says Laa ilaaha 
    ill-Allaah is a true believer, but he is tempted by his whims and 
    desires, so he does some acts of disobedience or major sins such as stealing 
    etc. 

    The belief of Ahl al-Sunnah wa'l-Jamaa'ah is that the destiny 
    of the believer, even if he commits a major sin, is Paradise. Whatever 
    punishment may come before Paradise is up to Allaah: if He wills, He will 
    punish him, and if He wills, He will forgive him. The evidence for that is 
    the verse in which Allaah says (interpretation of the meaning): 

    “Verily, Allaah forgives not that partners should be set 
    up with Him (in worship), but He forgives except that (anything else) to 
    whom He wills”

    [al-Nisa’ 4:48]

    So all those who committed sins – even if they were major 
    sins – if the sins were less than kufr, they will not be prevented from 
    entering Paradise. For the destiny of the one who did them is Paradise, but 
    he may be punished for the sins that he committed, or Allaah may forgive 
    him. The matter rests with Allaah. As for the hypocrites and followers of 
    bid’ahs which put a person beyond the pale of Islam, they did not really say
    Laa ilaaha ill-Allaah meaning it from the heart, because this 
    hypocrisy which led to kufr is the opposite of sincerity, and sincerity is 
    essential to saying Laa ilaaha ill-Allaah. 

    If a person says Laa ilaaha ill-Allaah, but he 
    believes that there is no God, or he believes that there is another god 
    alongside Allaah who controls the universe, or he believes that all the 
    companions of the Prophet  (peace and blessings of Allaah be upon him) 
    apostatized after his death, or other bid’ahs that put a person beyond the 
    pale of Islam, then such people are not sincere in their saying Laa 
    ilaaha ill-Allaah, because their bid’ah goes against the words of the 
    Prophet  (peace and blessings of Allaah be upon him): “Whoever bears 
    witness that there is no god except Allaah or says Laa ilaaha ill-Allaah, 
    will enter Paradise.” 

    The Shahaadatayn must be spoken with sincerity. Listen to 
    what Allaah says concerning the munaafiqeen (interpretation of the 
    meaning): 

    “And when they [the hypocrites] stand up for As-Salaah 
    (the prayer), they stand with laziness and to be seen of men, and they do 
    not remember Allaah but little”

    [al-Nisa’ 4:142]

    In the same passage, Allaah says (interpretation of the 
    meaning): 

    “Verily, the hypocrites will be in the lowest depth 
    (grade) of the Fire; no helper will you find for them”

    [al-Nisa’ 4:145]

    And Allaah says of them (interpretation of the meaning): 

    “When the hypocrites come to you (O Muhammad), they say: 
    ‘We bear witness that you are indeed the Messenger of Allaah’”

    [al-Munaafiqoon 63:1]

    This is merely verbal testimony, but: 

    “Allaah knows that you are indeed His Messenger, and 
    Allaah bears witness that the hypocrites are liars indeed”

    [al-Munaafiqoon 63:1 – interpretation of the meaning]

    i.e., that they are lying when they say, “We bear witness that you are indeed the Messenger of Allaah”. For they mention Allaah and bear witness to His Messenger, but their hearts are devoid of that which is uttered by their lips.