And if you doubt any part of what We have bestowed from on high, step by step, upon Our servant [Muhammad],14
then produce a surah of similar merit, and call upon any other than God to bear witness for you15
- if what you say is true!
I.e., the message of which the doctrine of God's oneness and uniqueness is the focal point. By the use of the word "doubt" (rayb), this passage is meant to recall the opening sentence of this surah: "This divine writ - let there be no doubt about it...", etc. The gradualness of revelation is implied in the grammatical form nazzalna - which is important in this context inasmuch as the opponents of the Prophet argued that the Qur'an could not be of divine origin because it was being revealed gradually, and not in one piece (Zamakhshari).
Lit., "come forward with a surah like it, and call upon your witnesses other than God" - namely, "to attest that your hypothetical literary effort could be deemed equal to any part of the Qur'an." This challenge occurs in two other places as well ( 10:38 and 11:13 , in which latter case the unbelievers are called upon to produce ten chapters of comparable merit); see also 17:88 .