The Quran

إِنَّ الدّينَ عِندَ اللَّهِ الإِسلامُ ۗ وَمَا اختَلَفَ الَّذينَ أوتُوا الكِتابَ إِلّا مِن بَعدِ ما جاءَهُمُ العِلمُ بَغيًا بَينَهُم ۗ وَمَن يَكفُر بِآياتِ اللَّهِ فَإِنَّ اللَّهَ سَريعُ الحِسابِ

Muhammad Asad

Behold, the only [true] religion in the sight of God is [man's] self-surrender unto Him; and those who were vouchsafed revelation aforetime¹ took, out of mutual jealousy, to divergent views [on this point] only after knowledge [thereof] had come unto them.² But as for him who denies the truth of God's messages - behold, God is swift in reckoning!

Most of the classical commentators are of the opinion that the people referred to are the followers of the Bible, or of parts of it - i.e., the Jews and the Christians. It is, however, highly probable that this passage bears a wider import and relates to all communities which base their views on a revealed scripture, extant in a partially corrupted form, with parts of it entirely lost.
I.e., all these communities at first subscribed to the doctrine of God's oneness and held that man's self-surrender to Him (islam in its original connotation) is the essence of all true religion. Their subsequent divergencies were an outcome of sectarian pride and mutual exclusiveness.