Behold, the only [true] religion in the sight of God is [man's] self-surrender unto Him; and those who were vouchsafed revelation aforetime12
took, out of mutual jealousy, to divergent views [on this point] only after knowledge [thereof] had come unto them.13
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.