Allaah (The Most High) said:
تَنزِيلٌ۬ مِّنَ ٱلرَّحۡمَـٰنِ ٱلرَّحِيمِ
كِتَـٰبٌ۬ فُصِّلَتۡ ءَايَـٰتُهُ ۥ قُرۡءَانًا عَرَبِيًّ۬ا لِّقَوۡمٍ۬ يَعۡلَمُونَ
”Ha-Mim. A revelation from Allah, the Most Beneficent, the Most Merciful. A Book whereof the Verses are explained in detail; A Qur’an in Arabic for people who know.” [41:1-3]
Allaah (The Most High) informed His slaves that indeed this lofty Book and beautiful Qur’aan is a [تَنزِيلٌ۬ ] revelation [مِّنَ ٱلرَّحۡمَـٰنِ ٱلرَّحِيم—from the Most Beneficent, the Most Merciful] Whose Mercy encompasses everything. And from His greatest and abundant mercy is the sending down of this Book through which knowledge and guidance, light and cure, mercy and abundant good are received. There is no greater favour of His upon the slaves than it, and it is the path towards happiness in both worlds (i.e. this life and the next).
Then He (Allaah) praised the Book saying that it gives a complete explanation: [فُصِّلَتۡ ءَايَـٰتُهُ]; that is: It explained every matter based on its differences and exact (perfect) determined boundaries, and this necessitates a complete explanation, distinction between everything and a distinction between the different affairs of reality.
[قُرۡءَانًا عَرَبِيًّ۬ا -A Qur’an in Arabic]: That is: In the pure (Arabic) language-the most perfect of languages. Its verses have been explained and revealed in the Arabic language [لِّقَوۡمٍ۬ يَعۡلَمُونَ- for people who know]; so that its meanings are made clear to them just as its wordings are clear; (and so that) guidance is made clearly distinct from misguidance and being misled is made distinct from being led aright.
As for the (obstinate) ignorant ones- those who (in the presence) of guidance are not increased in aught except misguidance; those who (in the presence) of clarification are not increased in aught except blindness, then speech is not uttered due to them; [سَوَآءٌ عَلَيۡهِمۡ ءَأَنذَرۡتَهُمۡ أَمۡ لَمۡ تُنذِرۡهُمۡ لَا يُؤۡمِنُونَ — It is the same to them whether you (O Muhammad) warn them or do not warn them, they will not believe.]
Slightly paraphrased…see Tayseer Al-Kareem ar-Rahmaan Fee Tafseer Kalaam al-Mannaan of Imaam Sadi (rahimahullaah)