In The Name of Allaah, The Most Merciful, The Bestower of Mercy
Allaah [The Most High] said:
تَنزِيلُ ٱلۡكِتَـٰبِ مِنَ ٱللَّهِ ٱلۡعَزِيزِ ٱلۡعَلِيمِ
غَافِرِ ٱلذَّنۢبِ وَقَابِلِ ٱلتَّوۡبِ شَدِيدِ ٱلۡعِقَابِ ذِى ٱلطَّوۡلِۖ لَآ إِلَـٰهَ إِلَّا هُوَۖ إِلَيۡهِ ٱلۡمَصِيرُ
Haa-Meem. The revelation of the Book (this Qur’an) is from Allah the All-Mighty, the All-Knower. The Forgiver of sin, the Acceptor of repentance, the Severe in punishment, the Bestower (of favours), Laa ilaaha illaa Huwa (none has the right to be worshipped but He), to Him is the final return. [Surah Ghaafir 1-3]
Allaah [The Most High] informs [us] about His Great Book – the Qur’aan – that it was from Him and sent down by Him; and that He alone has the right to be deified and worshipped due to the fact that He alone is the Perfect One [i.e. the only One described with all perfect Names and Attributes and He has no imperfections, faults, shortcomings, deficiencies, co-equals, similarities etc] and the One singled out in His Actions [i.e. He alone is the Creator, Provider, the one Who maintains the affairs of the universe in perfect order, the one in absolute control of life and death, and everything happens by His will etc]; [ٱلۡعَزِيزِ – The All-Mighty One], Who has absolute control over everything and He is never overcome; [ٱلۡعَلِيمِ – The All-Knower]; [غَافِرِ ٱلذَّنۢبِ – The Forgiver of sin]; [وَقَابِلِ ٱلتَّوۡبِ – the Acceptor of repentance]; [شَدِيدِ ٱلۡعِقَابِ- the Severe in punishment] on those who are unrepentant; and [ذِى ٱلطَّوۡلِۖ – the Bestower (of favours)].
So after Allaah [The Most High] established the fact that He alone is perfect, then this absolutely necessitates that He alone has the right to be deified and that all acts of worship are to be sincerely directed to Him; [لَآ إِلَـٰهَ إِلَّا هُوَۖ إِلَيۡهِ ٱلۡمَصِيرُ – none has the right to be worshipped but He), to Him is the final return].
The appropriateness of mentioning -in these Aayaat- that the Qur’aan was revealed by Allaah -[The One described with all perfect Names and Attributes, and free is He from all imperfections, shortcomings, faults, deficiencies, similarities, co-equals]- is because the Qur’aan comprises of all affairs mentioned in these Aayaat. The Qur’aan either informs [us] of the Perfect Names and Attributes of Allaah and His Actions or about the unseen events of the past, and what will take place in future- a lesson from Allaah [The All-Knower].
It either informs (us) of Allaah’s tremendous blessings and great favours bestowed on mankind, and this why Allaah [The Most High] said: [ذِى ٱلطَّوۡلِۖ – the Bestower (of favours)]; or about His severe punishment on those who commit sins [and are unrepentant], and this is why Allaah [The Most High] said:[شَدِيدِ ٱلۡعِقَابِ- the Severe in punishment]. It either informs (us) about the sinners being urged to repent, and this is why Allaah [The Most High] said: [غَافِرِ ٱلذَّنۢبِ وَقَابِلِ ٱلتَّوۡبِ شَدِيدِ ٱلۡعِقَابِ – The Forgiver of sin, the Acceptor of repentance, the Severe in punishment]; or about the fact that Allaah alone has the right to be deified and worshipped, whilst citing the clear rational and divine proofs to establish that; commanding [mankind and jinn] to worship Allaah alone and forbid them from worshipping others besides Him, whilst citing clear proofs to show the falsity of worshipping others besides Him and warning them about the [destructive consequences of polytheism in this life and the next], and this is why Allaah [The Most High] said: [لَآ إِلَـٰهَ إِلَّا هُوَۖ – none has the right to be worshipped but He (Allaah]. It also informs us of Allaah’s just recompense – the reward that will be granted to doers of good and the punishment that will be afflicted on the sinners, and this is why Allaah [The Most High] said: [ إِلَيۡهِ ٱلۡمَصِيرُ- to Him is the final return]. The Qur’aan gathers all these lofty, divine and perfect [commands and prohibitions related to every aspect of life, and by way of which mankind can live a life of purity, virtue, security, justice, perfect guidance in this worldly life, and a life of eternal bliss in the afterlife].
[Source: (Tayseer Al-kareem Ar-Rahmaan Fee Tafseer Kalaam Al-Mannaan). slightly paraphrased]