Some of them said by their tongues that they are Christians but actually their saying did not comply with their acts for they did not stand by Jesus (PBUH) side and did not support him, so they forgot what has revealed to them.

(… فَأَغْرَيْنَا بَيْنَهُمُ الْعَدَاوَةَ وَالْبَغْضَاءَ إِلَىٰ يَوْمِ الْقِيَامَةِ …) (…So We caused among them animosity and hatred until the Day of Resurrection.…): There is difference between animosity and hatred; animosity is interchanging enmity between two parties or persons while hatred may be from one side, that is you may hate a person while he does not know that, or does not feel same towards you. When they forgot a portion of that of which they were reminded, that is they disobey what Jesus (PBUH) has come with, Allah (swt) caused among them animosity and hatred so they divided.

(  وَسَوْفَ يُنَبِّئُهُمُ اللَّهُ بِمَا كَانُوا يَصْنَعُونَ … ) (And Allah is going to inform them about what they used to do.…): That will be on the Day of Resurrection, the day when the scales will be placed and the and the pages are made public.