Allah tells us that His Book, the Glorious Qur'an, has been sent with truth, i.e., it contains the truth, as Allah says:
لَّـكِنِ اللَّهُ يَشْهَدُ بِمَآ أَنزَلَ إِلَيْكَ أَنزَلَهُ بِعِلْمِهِ وَالْمَلَـئِكَةُ يَشْهَدُونَ
(But Allah bears witness to that which He has sent down unto you; He has sent it down with His knowledge, and the angels bear witness) 4:166 meaning, it contains the knowledge which Allah wanted to teach to you, with His rulings, commands and prohibitions.
وَبِالْحَقِّ نَزَلَ
(and with truth it has descended.) means, "It has been sent down to you, O Muhammad, preserved and protected, not contaminated or mixed with anything else, with nothing added or taken away. It has come to you with the truth, brought down by one mighty in power, trustworthy and strong, one who is obeyed by the higher group (angels).'
وَمَآ أَرْسَلْنَاكَ
(And We have sent you) O Muhammad
إِلاَّ مُبَشِّرًا وَنَذِيرًا
(as nothing but a bearer of glad tidings and a warner). a bearer of glad tidings for the believers who obey you and a warner to the disbelievers who disobey you.
وَقُرْءانًا فَرَقْنَاهُ
(And (it is) a Qur'an which We have divided), The word translated here as "We have divided" may be read in two ways. If it is read as "Faraqnahu", with no Shaddah, the meaning is: `We have made it depart from Al-Lawh Al-Mahfuz to Bayt Al-`Izzah in the lowest heaven, then it was revealed in stages to the Messenger of Allah ﷺ, according to events, over a period of twenty-three years.' This was narrated by `Ikrimah from Ibn `Abbas. It was also narrated that Ibn `Abbas read it as "Farraqnahu", with a Shaddah, meaning, `We revealed it Ayah by Ayah , and have explained it and made it clear.' Hence Allah says:
لِتَقْرَأَهُ عَلَى النَّاسِ
(in order that you might recite it to men), meaning, convey it to the people and recite it to them,
عَلَى مُكْثٍ
(at intervals.) meaning slowly.
وَنَزَّلْنَـهُ تَنْزِيلاً
(And We have revealed it by stages.) means, little by little.