وَأَنِ احْكُم بَيْنَهُم بِمَأ أَنزَلَ اللّهُ وَلَا تَتَّبِعْ أَهْوَاءهُمْ
And so judge between them by what Allah has revealed and follow not their vain desires,
emphasizes this command and forbids ignoring it.
Allah said next,
وَاحْذَرْهُمْ أَن يَفْتِنُوكَ عَن بَعْضِ مَا أَنزَلَ اللّهُ إِلَيْكَ
but beware of them lest they turn you far away from some of that which Allah has sent down to you.
meaning; beware of the Jews, your enemies, lest they distort the truth for you in what they convey to you. Therefore, do not be deceived by them, for they are liars, treacherous and disbelievers.
فَإِن تَوَلَّوْاْ
And if they turn away,
from the judgment that you pass in their disputes, and they defy Allah's Law.
فَاعْلَمْ أَنَّمَا يُرِيدُ اللّهُ أَن يُصِيبَهُم بِبَعْضِ ذُنُوبِهِمْ
then know that Allah's will is to punish them for some sins of theirs.
meaning, know that this will occur according to the decree of Allah, and because out of His wisdom they have deviated from the truth, and because of their previous sins.
وَإِنَّ كَثِيرًا مِّنَ النَّاسِ لَفَاسِقُونَ
And truly, most men are rebellious.
Therefore, the majority of humans are disobedient to their Lord, defiant of the truth and deviate away from it.
Allah said in other Ayat,
وَمَأ أَكْثَرُ النَّاسِ وَلَوْ حَرَصْتَ بِمُوْمِنِينَ
And most people will not believe even if you desire it eagerly. (12;103)
and,
وَإِن تُطِعْ أَكْثَرَ مَن فِى الاٌّرْضِ يُضِلُّوكَ عَن سَبِيلِ اللَّهِ
And if you obey most of those on the earth they will mislead you far away from Allah's path. (6;116)
Muhammad bin Ishaq reported that Ibn Abbas said,
"Ka`b bin Asad, Ibn Saluba, Abdullah bin Surya and Shas bin Qays said to each other, `Let us go to Muhammad to try and misguide him from his religion.'
So they went to the Prophet and said, `O Muhammad! You know that we are the scholars, noblemen and chiefs of the Jews. If we follow you, the Jews will follow suit and will not contradict us. But, there is enmity between us and some of our people, so we will refer to you for judgment in this matter, and you should rule in our favor against them and we will believe in you.'
The Messenger of Allah refused the offer and Allah sent down these Ayat about them,
وَأَنِ احْكُم بَيْنَهُم بِمَأ أَنزَلَ اللّهُ وَلَا تَتَّبِعْ أَهْوَاءهُمْ وَاحْذَرْهُمْ أَن يَفْتِنُوكَ عَن بَعْضِ مَا أَنزَلَ اللّهُ إِلَيْكَ
And so judge between them by what Allah has revealed and follow not their vain desires, but beware of them lest they turn you far away from some of that which Allah has sent down to you.) until,
لِّقَوْمٍ يُوقِنُونَ
(for a people who have firm faith.)" (5;49-50)
Ibn Jarir and Ibn Abi Hatim recorded this Hadith.
Allah continues,
أَفَحُكْمَ الْجَاهِلِيَّةِ يَبْغُونَ وَمَنْ أَحْسَنُ مِنَ اللّهِ حُكْمًا لِّقَوْمٍ يُوقِنُونَ