In The Name of Allaah, The Most Merciful, The Bestower of Mercy.

Allaah [The Most High] 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.” [Soorah Al-An’aam: Ayah 112]

Allaah [Glorified be He and free is He from all imperfections] stated that they aid one another in opposing the command of the Prophets with what some of them inspire others of adorned speech, so that the gullible and feeble minded people are misled by it.  He [The Most High] stated the reason behind a perpetrator’s [adorned speech] and the reason why it is accepted by the people; then He [The Most High] mentioned [in ayah 113 Al-An’aam] the effects it has on those ignorant souls. When those souls incline towards such adorned speech and become pleased with it, they carry out what it calls them to do -in speech and action.  Therefore, ponder upon these verses and the sublime meanings they convey, for they clarify the roots of false principles and alert a person against them. And if you ponder upon the statements of the people of falsehood, you will indeed find them clothed with expressions and presented in a good light with sophisticated terms, which are quickly accepted by those deprived of sound discernment-to the extent that the wicked sinners would give [good] names to the greatest types of wicked deeds. [1]

Allaah [The Exalted] said:

أَفَمَن زُيِّنَ لَهُ سُوءُ عَمَلِهِ فَرَآهُ حَسَنًا ۖ فَإِنَّ اللَّهَ يُضِلُّ مَن يَشَاءُ وَيَهْدِي مَن يَشَاءُ ۖ فَلَا تَذْهَبْ نَفْسُكَ عَلَيْهِمْ حَسَرَاتٍ ۚ إِنَّ اللَّهَ عَلِيمٌ بِمَا يَصْنَعُونَ

Is he, then, to whom the evil of his deeds made fair-seeming, so that he considers it as good? Verily, Allah sends astray whom He wills, and guides whom He wills. So destroy not yourself (O Muhammad) in sorrow for them. Truly, Allah is the All-Knower of what they do! [Surah Al-Faatir. Aayah 8]

[أَفَمَن زُيِّنَ لَهُ سُوءُ – Is he, then, to whom the evil of his deeds made fair-seeming] -Meaning, his evil and repugnant deeds have been made fair-seeming to him by shaytaan and beautified in his eyes; [فَرَآهُ حَسَنًا- so that he considers it as good] -Meaning, considers his evil and repugnant deeds to be good like the one whom Allaah has guided to the straight path and upright religion, but are the two the same? The first one commits evil deeds, considers truth to be falsehood and falsehood to be truth, and the second person performs good deeds, considers truth to be truth and falsehood to be falsehood. However, guidance is in the hands of Allaah; [ فَإِنَّ اللَّهَ يُضِلُّ مَن يَشَاءُ وَيَهْدِي مَن يَشَاءُ ۖ فَلَا تَذْهَبْ نَفْسُكَ عَلَيْهِمْ حَسَرَاتٍ – Verily, Allah sends astray whom He wills, and guides whom He wills. So destroy not yourself (O Muhammad) in sorrow for them]- Meaning, the misguided ones to whom their evil deeds are made fair-seeming and shaytaan hinders them from the truth. Therefore, there is nothing obligated on you (O Muhammad) except to convey the message and there is nothing on you regarding their guidance, rather Allaah is the one who will recompense them for their deeds. [ إِنَّ اللَّهَ عَلِيمٌ بِمَا يَصْنَعُونَ- Truly, Allah is the All-Knower of what they do]. [2]

Ibn Abbaas said: “This ayah was revealed concerning Abu Jahl and the idolaters of Makkah”. Sa’eed Bin Jubair said: This was revealed concerning the people of religious innovations and evil desires”. Qataadah said, “And from them are the khawaarij who make lawful the spilling of the blood of the Muslims and confiscating their wealth. As for the people of the Kabaa’ir (i.e. those sinful Muslims who commit major sins), they are not from them (i.e. not from those whom the evil of their deeds are made fair-seeming) because they do not declare the major sins to be lawful. The one to whom doubts are made fair-seeming and is submerged in it, and his deeds are made fair-seeming to him, so that he considers it as good, shaytaan makes it fair-seeming to him through whispers”. [3]

[Ref 1: An Excerpt from As-Sawaa-iq al-Mursalah of Imaam Ibnul Qayyim: 2/438). Slightly paraphrased]
[Ref 2: An Excerpt from Tafseer As-Sadi. Slightly paraphrased]
[Ref 3: Tafseer Al-Baghawi 3/689]