The Quran

وَمِنَ الَّذينَ قالوا إِنّا نَصارىٰ أَخَذنا ميثاقَهُم فَنَسوا حَظًّا مِمّا ذُكِّروا بِهِ فَأَغرَينا بَينَهُمُ العَداوَةَ وَالبَغضاءَ إِلىٰ يَومِ القِيامَةِ ۚ وَسَوفَ يُنَبِّئُهُمُ اللَّهُ بِما كانوا يَصنَعونَ

Muhammad Asad

And [likewise,] from those who say, "Behold, we are Christians."¹ We have accepted a solemn pledge: and they, too, have forgotten much of what they had been told to bear in mind - wherefore We have given rise among them to enmity and hatred, [to last] until Resurrection Day:² and in time God will cause them to understand what they have contrived.

Thus the Qur'an elliptically rejects their claim of being true followers of Jesus: for, by wrongfully elevating him to the status of divinity they have denied the very essence of his message.
I.e., their going astray from the genuine teachings of Jesus - and thus from true faith in God - is the innermost cause of the enmity and hatred which has so often set the so-called Christian nations against one another and led to unceasing wars and mutual persecution.