Allah tells us how the idolators persisted in their misguidance and in not paying attention to the sins that they had committed in the past or what was to happen to them in the future, on the Day of Resurrection.
(And when it is said to them: "Fear of that which is before you, and that which is behind you...") Mujahid said, "This refers to sins." Others said it is the opposite.
(in order that you may receive mercy.) means, `so that, if you fear such things Allah will have mercy on you and will save you from His punishment.' The wording implies that they would not respond. Rather that they would turn away and ignore that, as Allah says:
(And never came an Ayah from among the Ayat of their Lord to them,) meaning, signs of Tawhid and the truth of the Messengers,
(but they did turn away from it,) means, they did not accept it or benefit from it.
(And when it is said to them: "Spend of that with which Allah has provided you,") means, when they are told to spend of that which Allah has provided to them on the poor and needy among the Muslims,
(those who disbelieve say to those who believe) means, about the believers who are poor, i.e., they say to those believers who tell them to spend on the needy,
(Shall we feed those whom, if Allah willed, He (Himself) would have fed) meaning, `those whom you are telling us to spend on, if Allah had wanted to, He would have made them independent and fed them from His provision, so we are in accordance with the will of Allah with regard to them.
(You are only in a plain error.) means, `by telling us to do that.'