And if they do not respond to you - then know that it [i.e., the Qur’ān] was revealed with the knowledge of Allāh and that there is no deity except Him. Then, would you [not] be Muslims?
The Messenger grieving by the Statements of the Polytheists, and His Gratification
This statement of Allah, the Exalted, to His Messenger comforted the worries that the polytheists were causing him due to their statements directed towards him. This is just as Allah says about them,
(And they say: "Why does this Messenger eat food, and walk about in the markets. Why is not an angel sent down to him to be a warner with him" Or; "(why) has not a treasure been granted to him, or why has he not a garden whereof he may eat" And the wrongdoers say: "You follow none but a man bewitched.") 25:7-8 Thus, Allah commanded His Messenger and guided him to not let these statements of theirs grieve his heart. Allah directed him to not let these statements prevent him, or deter him from calling them to Allah, both day and night. This is as Allah said,
The reason is that they were first asked to bring ten Surahs. They failed to do so. Then, to make their inability look more pronounced, it was said in the verse of Surah al-Baqarah quoted above: If you take the Qur'an to be a word of man, then, you too, bring no more than just one Surah being the like of it. But, despite this challenge of the Qur'an, made so easy for them, they could do nothing. It stood proved that the Holy Qur'an was a miracle, and the Word of Allah without any doubt. Therefore, at the end, it was said: , that is, 'would you now believe and obey, or would you prefer to remain heedless as before?'
Tazkirul Quran
Maulana Wahiduddin KhanOn ayahs 14–15
When the Prophet Muhammad started opposing the ascribing of partners to God (shirk) and called upon the people to accept the oneness of God (tawhid), his addressees became perturbed. This was because the people most adversely affected by his statements were their great ones—the great ones whose religion they had adopted and were proud to venerate. The position was that these leading lights of ancient Arabia had attained pre-eminence in their eyes through a historical process, while as yet no historical greatness was attached to the Prophet Muhammad. He appeared to them at that time as a man of no consequence. So, the people of Arabia were greatly upset that an ordinary man was making statements which made their leaders lose in credibility. Under these circumstances the thought crosses the preacher’s mind that he should at least temporarily give up being critical in his approach and present his message in a more palatable way. ‘You may (feel the inclination) to leave aside a part of what is revealed to you’ means this very portion of the revelations, which contained criticism of polytheistic beliefs. But, Almighty God wants to clarify everything. And if, as a result of fully clarifying the Truth, the preacher is made the butt of ridicule and opposed, this inimical reaction on the part of the addressees is the cost a man has to pay in this world for becoming the preacher of unadulterated Truth. The most certain proof of the veracity of God’s messenger is his inimitable discourse. Those who scorned the Prophet and who rejected the idea that this apparently ordinary man possessed that Truth, which even their great ones did not have, were told that they should not test the Prophet’s veracity on the basis of his material condition, but should appreciate that his missionary discourse was so great that neither they nor their leaders could produce the like of it. This inimitable distinction is a definite proof of the fact that the Prophet was speaking for and on behalf of God. In spite of this clear sign of the Prophet’s veracity, why are people still hesitating to become the obedient servants of God?
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.