They are the People of the Book who divided it into parts, believing in some of it and disbelieving in some of it. This refers to the saying of Allah Almighty, "Those who have made the Quran into portions" [Hijr 15:91].
حدثني زياد بن أيوب حدثنا هشيم أخبرنا أبو بشر عن سعيد بن جبير
هم أهل الكتاب جزءوه أجزاء فآمنوا ببعضه وكفروا ببعضه يعني قول الله تعالى {الذين جعلوا القرآن عضين}