In The Name of Allaah, The Most Merciful The Bestower of Mercy
Allaah (Glorified Be He and Free Is He From all Imperfections) said:
وَكَذَٲلِكَ جَعَلۡنَا لِكُلِّ نَبِىٍّ عَدُوًّ۬ا شَيَـٰطِينَ ٱلۡإِنسِ وَٱلۡجِنِّ يُوحِى بَعۡضُهُمۡ إِلَىٰ بَعۡضٍ۬ زُخۡرُفَ ٱلۡقَوۡلِ غُرُورً۬اۚ وَلَوۡ شَآءَ رَبُّكَ مَا فَعَلُوهُۖ فَذَرۡهُمۡ وَمَا يَفۡتَرُونَ
And so We have appointed for every Prophet enemies- Shayaateen (devils) among mankind and jinns, inspiring one another with adorned speech as a delusion (or by way of deception). If your Lord had so willed, they would not have done it, so leave them alone with their fabrications.’ [6:112]
Imaam as-Saa’di (rahimahullaah) said: ”Allaah (The Most High) stated in consolation to His Messenger Muhammad (sallal-laahu-alayhi-wasallam) that this is the way things are. We have appointed for you enemies who reject your call, wage war against you and are envious towards you. [يُوحِى بَعۡضُهُمۡ إِلَىٰ بَعۡضٍ۬ زُخۡرُفَ ٱلۡقَوۡلِ غُرُورً۬اۚ – Inspiring one another with adorned speech as a delusion]; that is: beautifying the falsehood they call one another to, and adorning it with expressions until they make it appear in the finest image, so that the foolish and stupid ones are deceived by it- and those who neither understand the reality of things nor their (real) meanings are made to comply with it; rather they are amazed by beautified statements and distorted expressions, so they believe that that which is truth is falsehood and that which is falsehood is the truth…..
He (rahimahullaah) also said: And from the Wisdom of Allaah in appointing enemies for the Prophets and supporters of falsehood calling to it, is so that the slaves are faced with trials and tests in order that the truthful one is differentiated from the liar, the one of sound intellect from the ignorant one, and the one with insight from the blind one.
[Source: Tayseer Al-Kalaam Ar-Rahmaan Fee Tafseeri Kalaam Al-Mannaan. Abridged & slightly paraphrased]