Or do they say, "He has invented it"? Say, "If I have invented it, you will not possess for me [the power of protection] from Allāh at all. He is most knowing of that in which you are involved. Sufficient is He as Witness between me and you, and He is the Forgiving, the Merciful."
(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 أَرَأَيْتُم مَّا تَدْعُونَ مِن دُونِ اللَّـهِ (Say, "Tell me about those whom you invoke instead of Allah,...46:4) In these verses, the claim of the polytheists about the existence of some other gods has been refuted by demanding a proof to substantiate this claim, because no claim can be accepted, neither rationally nor according to the religious principles, unless it is proved by concrete evidences to support it. Then dealing with all possible types of arguments, it has been proved that they do not have any evidence or proof in their favor, and that their insisting on such a baseless claim is nothing but deviation from the truth. The arguments have been classified by these verses in three types. One is a rational argument. Verse 4 negates any argument of this type in their favor by saying, أَرُونِي مَاذَا خَلَقُوا مِنَ الْأَرْضِ أَمْ لَهُمْ شِرْكٌ فِي السَّمَاوَاتِ "Show me what they have created of the earth; Or have they a share in (the creation of the heavens?" (46:4)
In ancient Arabia, the addressees of the Quran were too preoccupied by the greatness of their forebears to give its message any proper consideration. They said that it distressed them even to hear it, because it ran counter to the religion of their august predecessors and, on this account, they rejected it. But the Quran had another aspect to it and that was its literary supremacy. Every Arabic linguistic expert has felt that it is an extraordinary work. It is true that the works of certain human beings possess extraordinary literary value, but there is a limit to the merit of a composition by an ordinary mortal. Indeed, the literary majesty of the Qur’an is inconceivably greater than any possible feat of the human mind. In order, therefore, to lessen the importance of the Quran from this angle, its detractors held that it was magic, i.e. it was a feat of magical presentation and not a sublime rendering of facts. When opponents stoop to obduracy, any serious person would remain silent after saying, ‘The issue between you and me is now before God for His decision.’ This is not defeatism but a forward-looking strategy. If one remains silent before an obstinate person, one actually distances oneself from the dispute and that brings one’s opponent face to face with his own conscience, so that if there is any feeling of rectitude left in him, it should be aroused.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.