It says...

It says: “And We did not send before you any messenger or a prophet but when he desired, the Satan threw (some vanity) into his desire;…” But Allah never left His Messenger alone with the rush of these satanic vain temptations, and He vanished them all, and then, He strengthened His signs. This deed is very easy for Allah to do, because He is All-Knowing, All-Wise, and He is completely aware of all these plots and plans, and He knows well how to nullify them.

“So that He may make what the Satan casts a trial for those in whose hearts is disease and those whose hearts are hard, and verily the unjust are in a deep opposition.” Satan’s temptations are as a means for testing the hard-hearted and sick-hearted persons. The spiritual sickness and hard-heartedness are usually some suitable grounds for falling into the snare of seditions and temptations.

These satanic plots of the opponents are always used for the examination of both the aware believers and the obstinate disbelievers. That is why the Qur’an says: “So that He may make what the Satan casts a trial for those in whose hearts is disease and those whose hearts are hard…” Then the Qur’an implies that the cruel oppressors have abided in a serious hostility and opposition while they are far from reality and the Truth.

“And that those who have been given the knowledge may know that it is the truth from your Lord, so they may believe in it and their hearts may be lowly before it, and verily, Allah guides those who believe unto a straight path.” The true knowledge is that which gives man the ability of recognizing right from wrong (and it must not be some imitative and superficial matters learnt by heart).