أَمْ يَقُولُونَ افْتَرَاهُ
Or they say;"He has fabricated it."
They are referring to Muhammad.
Allah responds saying,
قُلْ إِنِ افْتَرَيْتُهُ فَلَ تَمْلِكُونَ لِي مِنَ اللَّهِ شَيْيًا
هُوَ أَعْلَمُ بِمَا تُفِيضُونَ فِيهِ كَفَى بِهِ شَهِيدًا بَيْنِي وَبَيْنَكُمْ
وَهُوَ الْغَفُورُ الرَّحِيمُ
Say;"If I have fabricated it, you have no power to support me against Allah..."
which means, `if I lie against Allah and falsely claim that He sent me when He actually had not, then He would punish me with a severe punishment. No one on the earth -- you or anyone else -- would then be able to protect me from Him.'
It is similar to Allah's saying,
قُلْ إِنِّى لَن يُجِيرَنِى مِنَ اللَّهِ أَحَدٌ وَلَنْ أَجِدَ مِن دُونِهِ مُلْتَحَداً إِلاَّ بَلَغاً مِّنَ اللَّهِ وَرِسَـلَـتِهِ
Say;"None can protect me from Allah, nor would I find a refuge except in Him. (My duty) is only to convey from Allah and (deliver) His Messages." (72;22-23)
Allah says,
وَلَوْ تَقَوَّلَ عَلَيْنَا بَعْضَ الاٌّقَاوِيلِ
لاأَخَذْنَا مِنْهُ بِالْيَمِينِ
ثُمَّ لَقَطَعْنَا مِنْهُ الْوَتِينَ
فَمَا مِنكُم مِّنْ أَحَدٍ عَنْهُ حَـجِزِينَ
And if he made up a false saying concerning Us, We surely would have seized him by the right hand. Then we would have cut off his life artery. And none of you could withhold Us from (punishing) him. (69;44-47)
Thus, Allah says here,
قُلْ إِنِ افْتَرَيْتُهُ فَلَ تَمْلِكُونَ لِي مِنَ اللَّهِ شَيْيًا
هُوَ أَعْلَمُ بِمَا تُفِيضُونَ فِيهِ كَفَى بِهِ شَهِيدًا بَيْنِي وَبَيْنَكُمْ
Say;"If I have fabricated it, still you have no power to support me against Allah. He knows best of what you say among yourselves concerning it (i.e. this Qur'an)! Sufficient is He as a witness between me and you!..."
This is a severe threat, intimidation and warning for them.
Allah then says,
وَهُوَ الْغَفُورُ الرَّحِيمُ
And He is the Oft-Forgiving, the Most Merciful.
This is an encouragement for them to repent and turn to Allah. It means, `despite all of this, if you turn back and repent, Allah will accept your repentance, pardon you, forgive you, and have mercy upon on.'
This is similar to Allah's saying in Surah Al-Furqan,
وَقَالُواْ أَسَـطِيرُ الاٌّوَّلِينَ اكْتَتَبَهَا فَهِىَ تُمْلَى عَلَيْهِ بُكْرَةً وَأَصِيلً
قُلْ أَنزَلَهُ الَّذِى يَعْلَمُ السِّرَّ فِى السَّمَـوَتِ وَالاٌّرْضِ إِنَّهُ كَانَ غَفُوراً رَّحِيماً
And they say;"(This Book is) legends of the ancients that he has written down, and they are dictated to him morning and afternoon."
Say;"It has been sent down by the One Who knows every secret within the heavens and the earth. Truly, He is ever Forgiving and Merciful." (25;5-6)
Allah then says