Al-Furqan: 9

(Observe how they coin comparisons for you): An expression of the enormity of the falsehoods they dared to utter, and an expression of astonishment at them. That is: observe how they have said strange things regarding you that defy reason—things that, due to their oddity, function like proverbs—and how they have invented for you those anomalous attributes and states that are far removed from reality.

(So they have gone astray and cannot find a way): That is, they remain bewildered in their straying, unable to find an argument against your prophethood upon which they can settle, even if it were false in itself. The first fa (so) is causative, and the object of ḍallū (they went astray) is not intended [as a specific destination]. The second fa is explanatory.

Alternatively: "So they have deviated from the path of Truth, and thus they cannot find a way that leads to It." For whoever becomes accustomed to using these falsehoods can hardly be guided to the use of true premises. In this reading, the fa in both instances is causative, and the object of ḍallū is intended.

Perhaps the first [interpretation] is more appropriate. The objective is to negate that what they have brought forth could be a blemish upon his prophethood—may Allah bless him and grant him peace—and to definitively negate that they possess anything capable of being a valid objection, in the most eloquent manner possible. For an objection against prophethood can only be valid if it strikes at the miracles that attest to it, and what they have brought forward does not accomplish that at all. And how could they ever attain what would accomplish that?