Condemned form of fanatical attachment
Holding individual fanaticism and zealotry for one’s party/group whether they were upon truth or falsehood was a must according to them.
Sheikh Muhammed bin Abdul Wahaab Aspects of the Day of Ignorance (Masail Jahiliyyah)
The condemned fanaticism is (1) when one persists upon falsehood even though he knows it is false, out of pride and stubbornness, it is (2) aiding an individual or a tribe upon truth or falsehood. This is from the characteristics of the people of ignorance.
يَـٰٓأَيُّہَا ٱلَّذِينَ ءَامَنُواْ كُونُواْ قَوَّٲمِينَ لِلَّهِ شُہَدَآءَ بِٱلۡقِسۡطِۖ وَلَا يَجۡرِمَنَّڪُمۡ شَنَـَٔانُ قَوۡمٍ عَلَىٰٓ أَلَّا تَعۡدِلُواْۚ ٱعۡدِلُواْ هُوَ أَقۡرَبُ لِلتَّقۡوَىٰۖ وَٱتَّقُواْ ٱللَّهَۚ إِنَّ ٱللَّهَ خَبِيرُۢ بِمَا تَعۡمَلُونَ
O you who believe! Stand out firmly for Allah and be just witnesses and let not the enmity and hatred of others make you avoid justice. Be just: that is nearer to piety, and fear Allah. Verily, Allah is Well Acquainted with what you do.
Do not let your hatred for some people cause you to be unjust with their rights, even if they are your enemies. So being fair and just is something required whether it is with your friends or enemies.
وَإِذَا قُلۡتُمۡ فَٱعۡدِلُواْ وَلَوۡ ڪَانَ ذَا قُرۡبَىٰۖ وَبِعَهۡدِ ٱللَّهِ أَوۡفُواْۚ
And whenever you speak, say the truth even if a near relative is concerned, and fulfil the covenant of Allah.
If he errors you should change his error and not agree with it. Instead, you should advise him.
﴿ يَـٰٓأَيُّہَا ٱلَّذِينَ ءَامَنُواْ كُونُواْ قَوَّٲمِينَ بِٱلۡقِسۡطِ شُہَدَآءَ لِلَّهِ وَلَوۡ عَلَىٰٓ أَنفُسِكُمۡ أَوِ ٱلۡوَٲلِدَيۡنِ وَٱلۡأَقۡرَبِينَۚ إِن يَكُنۡ غَنِيًّا أَوۡ فَقِيرً۬ا فَٱللَّهُ أَوۡلَىٰ بِہِمَاۖ فَلَا تَتَّبِعُواْ ٱلۡهَوَىٰٓ أَن تَعۡدِلُواْۚ وَإِن تَلۡوُ ۥۤاْ أَوۡ تُعۡرِضُواْ فَإِنَّ ٱللَّهَ كَانَ بِمَا تَعۡمَلُونَ خَبِيرً۬ا ﴾
O you who believe! Stand out firmly for justice, as witnesses to Allah, even though it be against yourselves, or your parents, or your kin, be he rich or poor, Allah is a better Protector to both. So follow not the lusts, lest you may avoid justice; and if you distort your testimony or refuse to give it, it, verily, Allah is Ever Well-Acquainted with what you do.
A person is obligated to be just with himself as well as with his relatives, friends and enemies.
Sheikh Fawzan Explanation of the Book ‘Aspects of the Day of Ignorance’
Ahlul-Hadith 