Allah tells us about the foolishness of the disbelievers' ignorant minds, when they said about the Qur'an:
إِنْ هَـذَا إِلاَّ إِفْكٌ
(This is nothing but a lie), meaning an untruth.
افْتَرَاهُ
(that he has invented,) meaning the Prophet .
وَأَعَانَهُ عَلَيْهِ قَوْمٌ ءَاخَرُونَ
(and others have helped him in it.) means, he asked other people to help him compile it. So Allah said:
فَقَدْ جَآءُوا ظُلْماً وَزُوراً
(In fact, they have produced an unjust wrong and a lie.) meaning, they are the ones who are telling a lie, and they know that it is false, for their own souls know that what they are claiming is not true.
وَقَالُواْ أَسَـطِيرُ الاٌّوَّلِينَ اكْتَتَبَهَا
(And they say: "Tales of the ancients which he has written down...") meaning, the ancients wrote them down, and he has copied it.
فَهِىَ تُمْلَى عَلَيْهِ
(and they are dictated to him) means, they are read or recited to him.
بُكْرَةً وَأَصِيلاً
(morning and afternoon. ) at the beginning and end of the day. Because this idea is so foolish and is so patently false, everyone knows that it is not true. It is known through Mutawatir reports and is a common fact that Muhammad ﷺ the Messenger of Allah never learned to read or write, either at the beginning or the end of his life. He grew up among them for approximately forty years, from the time he was born until the time when his mission began. They knew all about him, and about his honest and sound character and how he would never lie or do anything immoral or bad. They even used to call him Al-Amin (the Trustworthy One) from a young age, until his mission began, because they saw how truthful and honest he was. When Allah honored him with that which He honored him, they declared their enmity towards him and came up with all these accusations which any reasonable person would know he was innocent of. They were not sure what to accuse him of. Sometimes they said that he was a sorcerer, at other times they would say he was a poet, or crazy, or a liar. So Allah said:
انْظُرْ كَيْفَ ضَرَبُواْ لَكَ الاٌّمْثَالَ فَضَلُّواْ فَلاَ يَسْتَطِيعْونَ سَبِيلاً
(See what examples they have put forward for you. So they have gone astray, and never can they find a way.) (17:48) In response to their stubbornness, Allah says here:
قُلْ أَنزَلَهُ الَّذِى يَعْلَمُ السِّرَّ فِى السَّمَـوَتِ وَالاٌّرْضِ
(Say: "It has been sent down by Him Who knows the secret of the heavens and the earth".) meaning, He has revealed the Qur'an which includes true information about the earlier and later generations, information which concurs with the realities of the past and future.
الَّذِى يَعْلَمُ السِّرَّ
(Who knows the secret) means, Allah is the One Who knows the unseen in the heavens and on the earth; He knows their secrets just as He knows what is visible therein.
إِنَّهُ كَانَ غَفُوراً رَّحِيماً
(Truly, He is Oft-Forgiving, Most Merciful.) This is an invitation to them to repent and turn back to Allah, telling them that His mercy is vast and His patience is immense. Whoever repents to Him, He accepts his repentance. Despite all their lies, immorality, falsehood, disbelief and stubbornness, and saying what they said about the Messenger and the Qur'an, He still invites them to repent and give up their sin, and to come to Islam and true guidance. This is like the Ayat:
لَّقَدْ كَفَرَ الَّذِينَ قَالُواْ إِنَّ اللَّهَ ثَـلِثُ ثَلَـثَةٍ وَمَا مِنْ إِلَـهٍ إِلاَّ إِلَـهٌ وَحِدٌ وَإِن لَّمْ يَنتَهُواْ عَمَّا يَقُولُونَ لَيَمَسَّنَّ الَّذِينَ كَفَرُواْ مِنْهُمْ عَذَابٌ أَلِيمٌ - أَفَلاَ يَتُوبُونَ إِلَى اللَّهِ وَيَسْتَغْفِرُونَهُ وَاللَّهُ غَفُورٌ رَّحِيمٌ
(Surely, disbelievers are those who said: "Allah is the third of the three." But there is no god but One God. And if they cease not from what they say, verily, a painful torment will befall on the disbelievers among them. Will they not turn with repentance to Allah and ask His forgiveness For Allah is Oft-Forgiving, Most Merciful.) (5:73-74)
إِنَّ الَّذِينَ فَتَنُواْ الْمُؤْمِنِينَ وَالْمُؤْمِنَـتِ ثُمَّ لَمْ يَتُوبُواْ فَلَهُمْ عَذَابُ جَهَنَّمَ وَلَهُمْ عَذَابُ الْحَرِيقِ
(Verily, those who put into trial the believing men and believing women, and then do not turn in repentance, then they will have the torment of Hell, and they will have the punishment of the burning Fire.) (85:10) Al-Hasan Al-Basri said: "Look at this kindness and generosity! They killed His friends and He is calling them to repentance and mercy."