Is it permissible for ordinary people to label as a kaafir one who reviles Islam, without referring to the scholars concerning that?
Question

                Is it permissible for ordinary people, if they hear someone reviling Allah or Islam or the Messenger (blessings and peace of Allah be upon him) to label him a kaafir without referring to the scholars?.
Praise be to Allah.
    Firstly: 

    There is no doubt that reviling Allah, may He be exalted – 
    Allah forbid – or reviling His Messenger or His religion – Allah forbid – 
    comes under the heading of disbelief in Allah the Almighty. The one who does 
    that has committed major kufr that puts one beyond the pale of Islam. If he 
    dies in that state without having repented from it, then he will be one of 
    the people of Hell who will abide therein for all eternity. 

    Shaykh Ibn Baaz (may Allah have mercy on him) said: 

    The scholars are unanimously and definitively agreed that if 
    a Muslim reviles Islam, or criticises it, or he reviles the Messenger 
    (blessings and peace of Allah be upon him), or criticises or mocks him, then 
    he is an apostate and a kaafir and it is permissible to sentence him to 
    death and seize his wealth.

    End quote from Fataawa Noor ‘ala ad-Darb by Ibn Baaz, 
    p. 139 

    Shaykh Ibn Jibreen (may Allah have mercy on him) was asked:

    What is the ruling on one who reviles Allah and His Religion? 
    What is the expiation for that? Please note that the man concerned is 
    married; does his wife become haraam for him or is she divorced? 

    He replied:

    There is no doubt that this is apostasy from Islam and 
    disbelief in Allah for which the person who does it deserves to be sentenced 
    to death unless he repents; his wife is divorced from him and his ties of 
    kinship with his relatives are severed, so he does not inherit from them and 
    they do not inherit from him. But if he repents, regrets it, prays for 
    forgiveness and acknowledges his mistake, Allah will accept his repentance 
    and he can take his wife back if her ‘iddah has not ended; if it has ended 
    then it is up to her, and it is not permissible for him to take her back 
    except with her consent.

    End quote from Fataawa Islamiyyah, 3/533 

    Secondly: 

    If a person hears someone clearly reviling Allah, or shar‘i 
    proof is established to that effect, then there is nothing wrong with him 
    believing that this person is a kaafir. This is the most abhorrent and 
    serious kind of reviling, and no one does that except one who has gone far 
    astray and has no respect for Allah, may He be glorified and exalted, or one 
    who has lost his mind and does not know what he is saying. 

    This kind of reviling is not one of the obscure issues that 
    need a ruling from the scholars or need to be subjected to examination; 
    rather it is something very clear and obvious that can be recognised by 
    ignorant and knowledgeable alike, and old and young will be disgusted by 
    it. 

    But at the same time it is essential to bear in mind the 
    shar‘i purpose behind denouncing that and regarding it as repugnant, which 
    is to remove and ward off that evil and to strive to make the one who said 
    it repent and come back to his Lord, even if he has apostatised and gone 
    beyond the pale of Islam. Asking the apostate to repent is something that is 
    well known and well established. So efforts should be made to exhort him in 
    a manner that is appropriate to his situation, explaining the abhorrent 
    nature of what he has done and to discuss with him the shar‘i way that is 
    appropriate to his situation. 

    The scholars of the Standing Committee said: 

    Reviling Islam – Allah forbid – is kufr according to the 
    texts and scholarly consensus. Allah, may He be exalted, said 
    (interpretation of the meaning):

    “Was it at Allah, and His Ayat (proofs, evidences, verses, 
    lessons, signs, revelations, etc.) and His Messenger (SAW) that you were 
    mocking?

    Make no excuse; you have disbelieved after you had 
    believed”

    [at-Tawbah 9:65].

    And there are other similar texts. 

    It is essential to advise him and denounce him for that; if 
    he responds, praise be to Allah, otherwise it is not permissible to initiate 
    the greeting of salaam with one who has reviled Islam, or to return his 
    greeting if he initiates the greeting; it is not permissible to accept his 
    invitation and he must be shunned completely until he repents or until the 
    ruling of Allah, which is to be sentenced to death, is carried out on him by 
    the authorities, because the Prophet (blessings and peace of Allah be upon 
    him) said: “Whoever changes his religion, execute him.” Narrated by 
    al-Bukhaari in his Saheeh, 3017.

    End quote from Fataawa al-Lajnah ad-Daa’imah, 2/12 

    Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said:

    If a person repents from any sin, even if that was reviling 
    Islam, his repentance will be accepted, if he fulfils the conditions that we 
    have mentioned. However it should be noted that a word may constitute kufr 
    or apostasy, but the one who utters it does not (necessarily) become a 
    kaafir thereby, because of the presence of some factor that prevents him 
    being labelled as a kaafir. In the case of this man who said that he reviled 
    Islam in a moment of anger, we say to him: If your anger was so severe that 
    you did not know what you were saying, and you did not know whether you were 
    in the sky or on the ground, and you said something without thinking and not 
    knowing what you were saying, there is no ruling that is applicable to these 
    words and you are not to be labelled an apostate, because this is something 
    that was said involuntarily and without thinking, and if anything is said 
    involuntarily and without meaning to, Allah, may He be glorified and 
    exalted, will not bring one to account for it. Allah, may He be exalted, 
    says concerning vows (interpretation of the meaning): “Allah will not 
    call you to account for that which is unintentional in your oaths, but He 
    will call you to account for that which your hearts have earned” [al-Baqarah 
    2:225].

    End quote from Fataawa Noor ‘ala ad-Darb, 24/2 

    For more information on the guidelines on labelling anyone as 
    a kaafir, please see the answer to question no.
    85102 

    And Allah knows best.