In ayah 46, Allah subhanahu wa ta’ala tells us that when Prophet Eesa aalyhi salaam came, he confirmed the previous Scripture i.e. Torah. However, he was not supposed to judge according to the Torah. He was given Injeel which was guidance and light just like Torah as mentioned in ayah 44.
وَقَفَّيْنَا عَلَى ءاثَـرِهِم بِعَيسَى ابْنِ مَرْيَمَ مُصَدِّقاً لِّمَا بَيْنَ يَدَيْهِ مِنَ التَوْرَاةِ وَءَاتَيْنَـهُ الإِنجِيلَ فِيهِ هُدًى وَنُورٌ وَمُصَدِّقاً لِّمَا بَيْنَ يَدَيْهِ مِنَ التَّوْرَاةِ وَهُدًى وَمَوْعِظَةً لِّلْمُتَّقِينَ
“And in their footsteps, We sent `Isa, son of Maryam, confirming the Tawrah that had come before him, and We gave him the Injil, in which was guidance and light and confirmation of the Tawrah that had come before it, a guidance and an admonition for those who have Taqwa.”
In the footsteps of the previous Prophets, the Rabbaniyyoon and the Ahbar, Allah subhanahu wa ta’ala sent Prophet Eesa who confirmed Torah and brought Injeel.
Allah subhanahu wa ta’ala sent Torah so that people could live by it. He then sent Prophet Eesa and with him He sent the Injeel. This means that each Book was for a specific era only, now the new Scripture had to be followed.
The Prophets confirmed what the previous Prophet had brought meaning they supported one another. This should be the attitude of the followers of the religion too. When we are following the same message then we should support one another.
Then Allah subhanahu wa ta’ala says in ayah 47,
وَلْيَحْكُمْ أَهْلُ الإِنجِيلِ بِمَآ أَنزَلَ اللَّهُ فِيهِ وَمَن لَّمْ يَحْكُم بِمَآ أَنزَلَ اللَّهُ فَأُوْلَـئِكَ هُمُ الْفَـسِقُونَ
“Let the people of the Injeel judge by what Allah has revealed therein. And whosoever does not judge by what Allah has revealed, such are the rebellious.”
When Injeel was sent then people were ordered to follow Injeel now and not Torah. Following a Book means implementing its teachings. If the Christians were really the followers of Injeel they would have accepted Prophet Muhammad sallAllahu aalyhi wa sallam as the Messenger of Allah and not defied him.
The previous ayaat have been ending at fasiqoon, zalimoon and kafiroon. This means anyone who defies the law of Allah or does not implement it then he is a fasiq [rebel], zalim [unjust] and kafir [disbeliever]. By not implementing the Qur’an or living our lives accordingly, we can’t call ourselves Muslims.