Overall meaning : Never have We sent a single prophet or messenger before you, Muhammad, but that when he recited Our revelations Satan would insinuate doubts and evil suggestions into his recitation in his attempt to turn people away from what he recited. But Allah would make null and void Satan’s insinuations and evil whisperings and would firmly establish His clear revelations in terms of both their wording and meaning. Allah is All-Knowing, All-Wise.
Allah makes Satan’s insinuations and evil whisperings a test for the hypocrites in whose hearts there is doubt and hypocrisy as well as the idolaters whose hearts are so hardened that they are never affected by admonitions. These wrongdoers have strayed far away from the truth and are strongly opposed to Allah and His Messenger.
Allah has also permitted this so that those who are endowed with knowledge, whereby they distinguish truth from falsehood, may know for sure that the Qur’an that He has sent down to you, Muhammad, is the truth which can in no way be approached by Satan, and so their faith in Allah may increase and their hearts may be humbled to Him. Allah guides those who believe in Him and His Messenger to the clear path of truth, Islam, with which He protects them against falling into error.