Surah Al Ma'idah Tafseer

وَلْيَحْكُمْ أَهْلُ الاِنجِيلِ بِمَا أَنزَلَ اللّهُ فِيهِ

Let the people of the Injil judge by what Allah has revealed therein.

meaning, so that He judges the people of the Injil by it in their time.

Or, the Ayah means, so that they believe in all that is in it and adhere to all its commands, including the good news about the coming of Muhammad and the command to believe in and follow him when he is sent.

Allah said in other Ayat,

قُلْ يَا أَهْلَ الْكِتَابِ لَسْتُمْ عَلَى شَيْءٍ حَتَّىَ تُقِيمُواْ التَّوْرَاةَ وَالاِنجِيلَ وَمَا أُنزِلَ إِلَيْكُم مِّن رَّبِّكُمْ

Say "O People of the Scripture! You have nothing (guidance) until you act according to the Tawrah, the Injil, and what has been sent down to you from your Lord." (5:68)

and,

الَّذِينَ يَتَّبِعُونَ الرَّسُولَ النَّبِيَّ الاُمِّيَّ الَّذِي يَجِدُونَهُ مَكْتُوبًا عِندَهُمْ فِي التَّوْرَاةِ

Those who follow the Messenger, the Prophet who can neither read nor write whom they find written with them in the Tawrah... until,

successful. (7:157)

Here, Allah said,

وَمَن لَّمْ يَحْكُم بِمَا أَنزَلَ اللّهُ فَأُوْلَـيِكَ هُمُ الْفَاسِقُونَ

And whosoever does not judge by what Allah has revealed, such are the rebellious.

meaning, the rebellious and disobedient of Allah who prefer falsehood and abandon truth.

We mentioned before that this Ayah was revealed about the Christians, and this is evident from the context of the Ayah