Ruling by other then Allah's Laws
Shaykh ul-Islam Ibn Taymiyyah stated in Majmoo’ al-Fataawa, vol.7, p.312: “Therefore, from the statements of the Salaf were: “A person can have within him both emaan and kufr” and likewise within their saying: “that one can have within him emaan and kufr, it is not the kufr which expels one from the religion.” Just as Ibn Abbas and his companions stated in regards to the saying of Allah, “And whoever does not judge by what Allah has revealed then it is those who are disbelievers.” (al-Maidah (5): 44) They said it means: “They disbelieved with a type of kufr which does not expel one from the religion”, and Ahmad and other Imams of the Sunnah followed him in that.”
Shaykh ul-Islam Ibn Taymiyyah also said in Majmoo’ al-Fataawa, vol.7, p.522: Ibn Abbas and others from the Salaf stated in regards to Allah’s saying, Whoever does not rule by what Allah has revealed, then they are the disbelievers.” {al-Maidah: 44} “Whoever does not rule by what Allah has revealed then they are the transgressors.”

{al-Maidah: 45} “Whoever does not rule by what Allah has revealed then they are the sinful.” {al-Maidah: 47} It is: “kufr less than kufr (which is minor kufr that does not expel a ruler from Islam), fisq less than fisq and dhulm less than dhulm. Imams Ahmad, al-Bukhari and others mentioned this.
Shaykh ul-Islam Ibn Taymiyyah also said in Majmoo’ al-Fataawa, vol.7, p.350-351: (the ruler who rules by other than Allahs law) he could be Muslim yet within him is kufr less than kufr which does not expel him from Islam absolutely, as the Companions said, such as Ibn Abbas and others: “kufr less than kufr” this is the saying of the generality of the Salaf and this is what Ahmad and others documented. This is also what al-Bukhari bore testimony to in his Saheeh. He intended by this(rahimahullaah) Imam al-Bukhari chaptering in his Saheeh: ‘Chapter: Kufr Towards Spouses and Kufr Less than Kufr.’ Al-Qaadee Ibn al-Arabee al-Maalikee stated in Fath ul-Baaree, vol.1, p.83: “. The intent of the classifier (i.e. al-Bukhari) is to explain that acts of obedience are named ‘emaan’ and likewise acts of disobedience are named ‘kufr’. However, when kufr is applied in this instance the intent is not the kufr which expels from the religion.