Allah (swt) does not mention the thing from which People of the Scripture and the polytheists were parted, because it is known from the context of speech. That is, those who disbelieved from the People of the Book and the polytheists were not parted from their disbelief and polytheism until after the evidence had come to them.
(disbelief) in Arabic means concealment or covering, hence, the meaning of (disbelieve) means to conceal. That is, those who have disbelieved are concealing the faith in Allah (swt) and His Messenger (saw).
مُنفَكِّينَ (to be parted): leaving what they were, that is, what they were of their disbelief and polytheism, as they are still in the same state.
الْبَيِّنَةُ (clear evidence): it is the clear evidence by which the right and wrong are differentiated. The evidence of light, strength, and instinct that Allah (swt) gives to the human being who thinks, so that he will be guided to the fact that behind this universe there is Aranger Creator. Thereof, the clear evidence is the instinct faith centered in the atoms of things.