In The Name of Allaah, The Most Merciful, The Bestower of Mercy.
And Allaah (The Most High) said:
لَّا تَجۡعَلُواْ دُعَآءَ ٱلرَّسُولِ بَيۡنَڪُمۡ كَدُعَآءِ بَعۡضِكُم بَعۡضً۬اۚ قَدۡ يَعۡلَمُ ٱللَّهُ ٱلَّذِينَ يَتَسَلَّلُونَ مِنكُمۡ لِوَاذً۬اۚ فَلۡيَحۡذَرِ ٱلَّذِينَ يُخَالِفُونَ عَنۡ أَمۡرِهِۦۤ أَن تُصِيبَہُمۡ فِتۡنَةٌ أَوۡ يُصِيبَہُمۡ عَذَابٌ أَلِيمٌ
Make not the calling of the Messenger [Muhammad] among you as your calling of one another. Allah knows those of you who slip away under shelter [of some excuse without taking the permission to leave, from the Messenger]. And let those who oppose the Messenger’s [Muhammad] commandment [i.e. his Sunnah legal ways, orders, acts of worship, statements, etc.] [among the sects] beware, lest some Fitnah befall them or a painful torment be inflicted on them. [Surah An-Noor. Aayah 63]
[فَلۡيَحۡذَرِ ٱلَّذِينَ يُخَالِفُونَ عَنۡ أَمۡرِهِۦۤ -And let those who oppose the Messenger’s (Muhammad) commandment (i.e. his Sunnah, orders, acts of worship, statements) beware]; [أَن تُصِيبَہُمۡ فِتۡنَةٌ – lest some Fitnah befall them]: Meaning: [Lest] misguidance, calamity in the worldly life or disbelief[Ref a] befall them. [أَوۡ يُصِيبَہُمۡ عَذَابٌ أَلِيمٌ – or a painful torment be inflicted on them]: Meaning: Killed [i.e. by enemies] in the worldly life or punished in the hell fire in the afterlife. [Ref 1]
In these noble verses, Allaah [The Most High] commanded (us) to obey Him and His Messenger, and He made obedience to Him and His Messenger a means to happiness in this life and the next. And He forbade (us) from disobeying Him and His Messenger, and He made disobedience to Him and His Messenger a cause of punishment in this worldly life- a punishment [whose severity] cannot be perceived. And because of this Allaah ended these two verses by mentioning Fitnah [i.e. trial (as an indication of the)] severity of His Punishment and the painful torment (afflicted on) the one who disobeys Him and His Messenger.
Imaam Ahmad [rahimahullaah] said about this Ayah [i.e. Surah An-Noor. Aayah 63]: Do you know what Al-Fitnah is? Al-Fitnah is Shirk. Perhaps if he [i.e. the person] rejects some of his [i.e. the Prophet’s] statements, then something of deviation enters his heart, so he is destroyed. [Reported by Ibn Battah in Al-Ibaanah Al-Kubraa 97]
The punishments [that occur due to] sins and disobedience do vary depending on the level of the offences [committed] – sometimes the punishment occurs [as trial] in one’s religion and this is the greatest punishment. It is [when] the act of disobedience comes between a person and Imaan [Ref 2] and between him and Husnul khaatimah. [Ref 3]
If every single one of us scrutinizes himself attentively, he will find that whenever he commits a sin and fails to repent, he will realise that he did commit a greater sin than it previously. And the testimony to this is the saying of Allaah: [فَلَمَّا زَاغُوٓاْ أَزَاغَ ٱللَّهُ قُلُوبَهُمۡۚ –So when they turned away (from the Path of Allah), Allah turned their hearts away (from the Right Path)]
Allaah (The Most High) said: [وَنُقَلِّبُ أَفۡـِٔدَتَہُمۡ وَأَبۡصَـٰرَهُمۡ كَمَا لَمۡ يُؤۡمِنُواْ بِهِۦۤ أَوَّلَ مَرَّةٍ۬ وَنَذَرُهُمۡ فِى طُغۡيَـٰنِهِمۡ يَعۡمَهُونَ – And We shall turn their hearts and their eyes away (from guidance), as they refused to believe therein for the first time, and We shall leave them in their trespass to wander blindly]. [6:110]
And Allaah (The Most High) said: [فَأَعۡقَبَہُمۡ نِفَاقً۬ا فِى قُلُوبِہِمۡ إِلَىٰ يَوۡمِ يَلۡقَوۡنَهُ ۥ بِمَآ أَخۡلَفُواْ ٱللَّهَ مَا وَعَدُوهُ وَبِمَا ڪَانُواْ يَكۡذِبُونَ – So He punished them by putting hypocrisy into their hearts till the Day whereon they shall meet Him, because they broke that (covenant with Allah) which they had promised Him and because they used to tell lies]. [9:77] [Ref 4]
[Ref a]: Takfeer and its principles: http://www.salafipublications.com/sps/sp.cfm?subsecID=MNJ09&articleID=MNJ090006&articlePages=1http://www.salafipublications.com/sps/sp.cfm?subsecID=MNJ09&articleID=MNJ090004&articlePages=1http://www.salafipublications.com/sps/sp.cfm?subsecID=MNJ09&articleID=MNJ090003&articlePages=1http://www.salafipublications.com/sps/sp.cfm?subsecID=MNJ09&articleID=MNJ090001&articlePages=1http://www.salafipublications.com/sps/sp.cfm?subsecID=MNJ09&articleID=MNJ090005&articlePages=1
Ref 1: Zaadul Maseer Fee Ilmit Tafseer’ By Imaam Ibnul Jawzi [rahimahullaah]
Ref 2: An act of disobedience comes between a person and Imaan: Imaam Sadi (rahimahullaah) stated about the Ayah: [وَاعْلَمُوا أَنَّ اللَّهَ يَحُولُ بَيْنَ الْمَرْءِ وَقَلْبِهِ – And know that Allah comes in between a person and his heart]- Therefore, beware of rejecting the command of Allaah when it comes to you at first, lest He comes between you and it [i.e. Allaah (The Most High) prevents you from guidance as a just recompense) when you turn away, for indeed Allaah intervenes between a person and his heart. Allaah causes it (i.e. the heart of a person) to change and turns it in any direction as He wills (i.e. as a just recompense for the one who rejects or turns away from the truth). Therefore, a slave (i.e. a Muslim) should increase in (uttering) the statement (i.e. the Du’aa): “O turner of the hearts, keep my heart firm upon your Religion; O turner of the hearts, turn my heart towards obedience to You.” [For further details, see Tayseer Al-Kareem Ar-Rahmaan Fee Tafseer Kalaam Al-Mannaan] [Also see Tafseer of this ayah in Al-Fawaa-id of Imaam Ibnul Qayyim (rahimahullaah) pages 140-144. Publishers: Maktabah Ar-Rushd. 3rd edition 2005]
[Ref 3]: Husnul Khaatimah: (i.e. It is when a Muslim leaves this world in obedience to Allaah and in obedience to Allaah’s Messenger; or leaves this world in a state confirmed to be a good end by textual proof. Imaam Albaani (rahimahullaah) stated some of these good signs in Ahkaam Al-Janaa-iz and from them: [a] The one who dies whilst uttering the Shahaadah; [b] The one who dies during the day or night on Friday; [c] Death by way of a plague, drowning etc; [d] The woman who dies during the period of post-natal bleeding as a result of giving birth; [e] death by fire; death through defending one’s wealth against a robber etc [See Ahkaam Al-Janaa’iz pages 21-24 –Shaikh Albaani (rahimahullaah) quotes the authentic ahaadeeth to establish these affairs and others]
[Ref 4]: Source: عقوبات الله المتنوعة وأسبابُه’ by Shaikh Abdullaah Ibn Salfeeq Adh-Dhufayree (may Allaah preserve him). Abridged and slightly paraphrased]. https://salaficentre.com/2015/08/allaahs-different-types-of-punishments-by-shaikh-abdullah-adhdhufairi-pdf/