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,
(Indeed, We know that your breast is straitened at what they say.) 15:97 Allah says in this verse,
(So perchance you may give up a part of what is revealed unto you, and that your breast feels straitened for it because they say...) The meaning here is that he (the Prophet ) may be compelled to give up the Message due to what they (the polytheists) say about him. However, Allah goes on to explain: "You (Muhammad) are only a warner and you have an example in your brothers of the Messengers who came before you. For verily, the previous Messengers were rejected and harmed, yet they were patient until the help of Allah came to them."
Then Allah, the Exalted, explains the miracle of the Qur'an, and that no one is able to produce its like, or even bring ten chapters, or one chapter like it. The reason for this is that the Speech of the Lord of all that exists is not like the speech of the created beings, just as His attributes are not like the attributes of the creation. Nothing resembles His existence. Exalted is He, the Most Holy, and the Sublime. There is no deity worthy of worship except He and there is no true Lord other than He. Then Allah goes on to say,
(If then they answer you not,) Meaning, that if they do not come with a reply to that which you have challenged them with (to the reproduction of ten chapters like the Qur'an), then know that it is due to their inability to do so. Know (that this is a proof) that this is the speech revealed from Allah. It contains His knowledge, His commands and His prohibitions. Then Allah continues by saying,
(and that there is no God besides Him! Will you then be Muslims)