And thus We have made for every prophet an enemy, devils (from among the) human beings and the jinn, some (of them) communicating to others (through) decoratively deceptive language, and had your Fosterer willed they would not have done it, so leave alone, them and that which they forge,
If there is no opposition there is no force. This appears to be the reason for appointing enemies to oppose the Prophets, the obvious result of which was a forceful propagation of the message of Allah (SWT) for instance the migration of Makkan Muslims to Madina, the religious battles and the final triumph of Islam. The enemies of Prophets misled infidels through attractive means because Allah (SWT) had willed so, therefore they are to be left alone to earn evil and the consequent punishment.