Praise be to Allah.
Kufr of indifference was defined by Ibn al-Qayyim (may Allah have mercy on him) as follows:
Kufr of indifference means turning away with one's hearing and one's heart from the Messenger, neither believing him nor disbelieving him, neither loving and supporting him nor hating him and opposing him, and not caring about what he brought at all.
End quote from Riyaad al-Saaliheen, 1/346
He also said:
Pure kufr of indifference means not paying attention to what the Messenger brought, neither loving and supporting him nor hating and opposing him; rather one is indifferent about both following him and opposing him. Miftaah Dar al-Sa‘aadah, 1/94
It was defined by Shaykh Muhammad ibn ‘Abd al-Wahhaab (may Allah have mercy on him), who listed it among the things that nullify Islam, as follows:
… Turning away from the religion of Allah, may He be exalted, and not learning it or acting upon it. End quote.
What Ibn al-Qayyim (may Allah have mercy on him) was speaking about was a kind of indifference which is the attitude of the kaafir, which prevents him from entering Islam; what Shaykh Muhammad ibn ‘Abd al-Wahhaab (may Allah have mercy on him) was speaking about has to do with the kaafir, which prevents him from following Islam in terms of learning it and acting upon what he learns. It may also have to do with the Muslim, in which case it amounts to apostasy from Islam, hence he regarded it as being one of the things that nullify Islam.
This thing that nullifies Islam has nothing to do with persisting in sin, which is less serious than shirk and kufr. We do not know of any way in which persisting in sin can be regarded as kufr.
What Ahl al-Sunnah wa’l-Jamaa‘ah believe is that the one who persists in sin is a faasiq (evildoer), not a kaafir (disbeliever), and that none of the people of the qiblah (Muslims) can be deemed a kaafir because of any sin less than shirk so long as he does not regard it as permissible.
Regarding haraam things as permissible is of two types:
1.Regarding what is haraam as halaal openly, such as stating clearly that what is haraam is halaal. A person becomes a kaafir thereby both outwardly and inwardly.
2.Regarding what is haraam as halaal inwardly, such as believing that everything that is haraam according to consensus is halaal, without having any reason to be confused about this issue, but without stating that clearly and openly. In this case a person becomes a kaafir with regard to what is between him and Allah, but the rulings on being a Muslim apply to him outwardly, because his kufr is not obvious. In this case he is like the hypocrite who is inwardly a kaafir although he appears to be a Muslim.
Persisting does not indicate that a person thinks something haraam is halaal. How many sinners have been overwhelmed by desires, so they persist in committing sin, but they believe that it is haraam and hate it in their hearts.
Al-Bukhaari narrated (6780) from ‘Umar ibn al-Khattaab (may Allah be pleased with him) that there was a man at the time of the Prophet (blessings and peace of Allah be upon him) whose name was ‘Abd-Allah and whose nickname was Himaar (donkey); he used to make the Prophet (blessings and peace of Allah be upon him) smile. The Prophet (blessings and peace of Allah be upon him) had him flogged for drinking. Then he was brought one day and the Prophet (blessings and peace of Allah be upon him) ordered that he be flogged (again). One of the people said: O Allah, curse him; how often he is brought (for punishment). The Prophet (blessings and peace of Allah be upon him) said: “Do not curse him, for by Allah, what I know is that he loves Allah and His Messenger.”
A person may know this for himself; he may have the problem of committing a sin that he commits regularly even though he believes that it is haraam and hopes to rid himself of it.
But what is feared for the one who persists in sin without repenting is that it may lead him to a bad end, Allah forbid, or he may begin to love the sin in which he persists, so that leads him to no longer feel in his heart what he is obliged to believe, which is that it is haraam. So he begins to regard the sin as permissible because he is used to it and loves it.
Hence one of the salaf said: Sin is the harbinger of kufr and there is the fear for the one who persists in sin that if he takes the matter lightly and does it as he does permissible things, without hating it or fearing the consequences, his faith will be rendered invalid.
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said: A man only commits haraam actions because of the weakness of his faith and love; if he does makrooh actions it is because he does not have enough hatred towards it, or because of the strength of the love for it that overwhelmed his heart. No person does any haraam action, such as immoral conduct both obvious and hidden, or sin and transgression, or associates things with Allah for which He has sent down no authority, or speaks about Allah without knowledge, except because of weakness of faith, because of lack of knowledge and belief, or because of the weakness of love and hate. But if his faith is basically sound, which means that he believes in all that Allah sent, then when the believer does these actions, he feels hatred and resentment towards them, and if he does them then it is because his whims and desires overwhelmed him, so when doing them he inevitably hates them and fears Allah's punishment for them, and he has the hope that he will rid himself of the consequences, either by repenting or by doing good deeds or by being forgiven, and so on. But if he does not hate them and does not fear Allah with regard to them and does not hope for His mercy, then this is not a believer under any circumstances; rather he is a kaafir or a hypocrite. End quote from Qaa‘idah fi’l-Mahabbah, p. 104
To sum up: persisting in sin is a major sin, but it does not constitute kufr unless it is accompanied by regarding the action as permissible. Persisting in it does not necessarily indicate that the person regards it as permissible, but it may lead to that. We ask Allah to keep us safe and sound.
And Allah knows best.