The Quran

وَما تَفَرَّقوا إِلّا مِن بَعدِ ما جاءَهُمُ العِلمُ بَغيًا بَينَهُم ۚ وَلَولا كَلِمَةٌ سَبَقَت مِن رَبِّكَ إِلىٰ أَجَلٍ مُسَمًّى لَقُضِيَ بَينَهُم ۚ وَإِنَّ الَّذينَ أورِثُوا الكِتابَ مِن بَعدِهِم لَفي شَكٍّ مِنهُ مُريبٍ

Ali Unal

And they (who follow the previous ways) were not divided (into opposing groups) until after the knowledge came to them (of the way they must follow and what that entailed, and only) because of envious rivalry and insolence among themselves. Had it not been for a decree already issued by your Lord (granting people respite and postponing the final, decisive judgment) until an appointed term, it would indeed have been judged between them. Surely those who (succeeded those opposing groups and) inherited the Book are in doubting uncertainty about it.¹

This verse mainly concerns the People of the Book, the followers of Moses and Jesus, upon them be peace. Their being in doubt about their Book means that some of them are really in doubt about its being a Divine Book and/or about some passages being really from God, while some others doubt whether it has been preserved and transmitted to them in its originality, and still others have other uncertainties about it. For the decree already issued by God, see sūrah 2: 36; sūrah 7: 24.