Allāh has enjoined upon us specific commands & prohibitions that every Muslim must abide to, & depending on the act, one who disobeys or acts in opposition to one of these commands or prohibitions is either a sinner or becomes a disbeliever. In relation to this subject, one of the more recent issues to take place amongst the Muslims is the inquisition: is one who rules by other than what Allāh has revealed a believer or a disbeliever?
Allāh says in al-Qurʾān:
وَمَن لَّمۡ يَحۡكُم بِمَآ أَنزَلَ ٱللَّهُ فَأُوْلَٰٓئِكَ هُمُ ٱلۡكَٰفِرُونَ
And whoever does not judge by what Allāh has revealed – then it is those who are the disbelievers.
– Sūrah al-Māʼidah, ʾĀyah 44
وَمَن لَّمۡ يَحۡكُم بِمَآ أَنزَلَ ٱللَّهُ فَأُوْلَٰٓئِكَ هُمُ ٱلظَّٰلِمُونَ
And whoever does not judge by what Allāh has revealed – then it is those who are the wrongdoers.
– Sūrah al-Māʼidah, ʾĀyah 45
وَمَن لَّمۡ يَحۡكُم بِمَآ أَنزَلَ ٱللَّهُ فَأُوْلَٰٓئِكَ هُمُ ٱلۡفَٰسِقُونَ
And whoever does not judge by what Allāh has revealed – then it is those who are the defiantly disobedient.
– Sūrah al-Māʼidah, ʾĀyah 47
Majority of people have taken these ʾĀyāt either out of context, or completely interpreted them contrary to what they actually entail; & these people are usually of 3 types:
- Those with similar beliefs to the Khawarij.
As for Ahl as-Sunnah wa al-Jamāʾah, they neither eradicate these ʾĀyāt from their proper context, nor interpret them to be other than what’s intended; rather, they scrutinize the texts in corroboration with it’s context & then interpret them according to al-Qurʾān & Sunnah based upon the understanding of the Salaf aṣ-Ṣāliḥ, & Ijmāʿ of the scholars; as ‘Ubaydullāh ibn Muslim ibn Yasār narrated from his father who said: “Before you speak about Allāh, pause and look at what precedes and follows it (i.e., look at the context).” 
In regards to the first ʾĀyah, Imām ibn Kathīr رحمه الله (d. 774 AH/1373 CE) says in his Tafsīr: “al-Barā‘ ibn ‘Azib, Ḥudhayfah ibn al-Yamān, ibn ʿAbbās, Abū Mijlaz, Abū Rajā‘ al-Uṭāridī, ʿIkrima, ʿUbaydullāh ibn ʿAbdullāh, Ḥasan al-Baṣrī, and others said that this ʾĀyah was revealed about the People of the Book. Ḥasan al-Baṣrī added that this ʾĀyah also applies to us. ʿAbd ar-Razzāq narrated from (Sufyān) ath-Thawrī, who narrated from Manṣūr, that Ibrāhīm said: ‘These ʾĀyāt were revealed about Banī ʾIsrāʾīl, and Allāh accepted them for this Ummah. ibn Jarīr (aṭ-Ṭabarī) recorded this statement.
ʿAlī ibn Abī Ṭalḥah also stated that ibn ʿAbbās said (concerning the ʾĀyah): ‘Whoever rejects what Allāh has revealed has committed Kufr, and whoever accepts what Allāh has revealed but did not rule by it is a Ẓālim (unjust), Fāsiq (sinner), and a criminal.’ ibn Jarīr (aṭ-Ṭabarī) recorded this statement.
ʿAbd ar-Razzāq said Ma’mar narrated from Ṭāwus that ibn ʿAbbās said (concerning the ʾĀyah): ‘It is an act of Kufr.’ ibn Ṭāwus added, ‘It is not like those who disbelieve in Allāh, His angels, His Books, and His Messengers.’ Waki’ narrated from Saʽīd al-Makkī that Ṭāwus said (concerning the ʾĀyah): ‘This is not the Kufr that annuls one’s religion.'” 
Shaykh Muḥammad ibn Ṣāliḥ al-‘Uthaymīn رحمه الله (d. 1421 AH/2001 CE) briefly explains: “It is incumbent upon the student of knowledge to know the difference between legislation that establishes a system which they leave and replace the Qurʾān with, and ruling in a specific matter based on other than what Allāh has revealed. This may amount to disbelief, sin, or wrong. It will be disbelief if he holds (the belief) that it is better than the rulings of the Sharīʿah or similar to it. It is sinfulness if it is owing to following desires regarding the particular matter, and it constitutes wrong if harming the followers is intended with that.” 
Shaykh Ṣāliḥ al-Fawzān حفظه الله speaks more on this issue: “Judging by other than what Allāh has revealed is disbelief; this disbelief is sometimes major disbelief, which takes one out of the fold of Islām, and at other times, it is minor disbelief, which does not remove one from the fold of Islām. This depends upon the condition of the judge; if he believes that judging by what Allāh has revealed is not obligatory and that he has a choice therein, contemptous of the Judgement of Allāh and believes that other laws such as constitutions and by-laws are better than or equivalent to it, believes that it is not applicable to present age, or he intends to please the disbelievers and hypocrites by judging by other than what Allāh has revealed, then this is major disbelief.
If however, he believes that judging by what Allāh has revealed is obligatory and he knows the judgement in the matter before him but turns away from it though acknowledging that he is liable to punishment (for turning away from it), then he is a sinner and he is a called a disbeliever who commits minor disbelief. If he is ignorant of the judgement of Allāh (regarding the matter) after making extensive effort to the extent of his capability to know the correct judgement thereof but errs, he is called one who has made an error. He gets a reward for his effort and his error is forgiven. These are in regards to judgements on specific issues.” 
As for judgement in matters of generality (for everybody), this is a different matter. Regarding this, Shaykh al-Islām ibn Taymīyyah رحمه الله (d. 728 AH/1328 CE) said: “If the judge is religious, but he judged without knowledge, then he is one of the inmates of the Fire. If he knows the truth but he judges in conflict with the truth of which he knows, then he is one of the inmates of the Fire. If he judges without justice and knowledge, then he is more deserving of being an inmate of the Fire. This is when he judges in a matter concerning a single person.
If he gives a general judgement in the religion of the Muslims and he made truth falsehood and made falsehood truth, Sunnah innovation and innovation Sunnah, good evil and evil good, forbade what Allāh and His Messenger commanded and ordered that which Allāh and His Messenger forbade, this is another case entirely which the Rabb of the universe, the God of the Messengers, the Owner of the Day of Judgement, and He Who owns all praise in the beginning and in the end, will judge.
لَهُ ٱلۡحُكۡمُ وَإِلَيۡهِ تُرۡجَعُونَ
His is the judgement, and to Him you will be returned.
– Sūrah al-Qaṣaṣ, ʾĀyah 88
هُوَ ٱلَّذِيٓ أَرۡسَلَ رَسُولَهُۥ بِٱلۡهُدَىٰ وَدِينِ ٱلۡحَقِّ لِيُظۡهِرَهُۥ عَلَى ٱلدِّينِ كُلِّهِۦۚ وَكَفَىٰ بِٱللَّهِ شَهِيدٗا
It is He who sent His Messenger with guidance and the religion of truth to manifest it over all religion. And sufficient is Allāh as Witness.
– Sūrah al-Fatḥ, ʾĀyah 28
[End quote.]” 
In summary, one who does not rule by what Allāh has revealed is classified into 3:
- They believe it is not obligatory or there is something better than it: such is ruled a disbeliever.
- They believe it is obligatory & there is nothing better than it but turns away due to desire knowing they’ll be punished: such is a sinner who commited minor disbelief.
- They are ignorant of Allāh’s judgement pertaining to a specific matter so they issue a judgement they thought was the closest to the Sharīʿah but errs: such is one who has made an error, which they will be rewarded & forgiven for.
Therefore, one who does not rule by what Allāh has revealed is not immediately declared a disbeliever, and Allāh knows best.
All praise is due to Allāh, may His peace & blessings be upon our final Prophet Muḥammad, his family, his companions, & all those who follow him in guidance.
 Muqaddimah fī Uṣūl at-Tafsīr (مقدمة في أصول التفسير) by Shaykh al-Islām ibn Taymīyyah | Page 79
 Tafsīr ibn Kathīr (تفسير ابن كثير) by Imām ibn Kathīr | Sūrah 5, ʾĀyah 44
 al-Qawl al-Mufīd Sharḥ Kitāb at-Tawḥīd (القول المفيد شرح كتاب التوحيد) by Shaykh Muḥammad ibn Ṣāliḥ al-‘Uthaymīn | Volume 2, Page 278
 ʿAqīdah at-Tawḥīd (عقيدة التوحيد) by Shaykh Ṣāliḥ al-Fawzān | Pages 174-175
 Majmūʾ al-Fatāwā (مجموعة الفتاوى) by Shaykh al-Islām ibn Taymīyyah | Volume 35, Page 388
Ṣafīullāh Labīb ibn Salīm ʿAbd al-Malik
4th of Rabīʿ al-ʾAwwal 1444