Tafsir of Al-Furqan 25:41

(And when they see you, they take you not except in mockery)—meaning: they do not treat you as anything other than an object of mockery, that is, a place of mockery or one mocked. Huzuwan (mockery) is an infinitive used in the sense of the passive participle for the sake of emphasis, or it is based on the estimation of an omitted possessor (mudaf).

The sentence (they take you not) is the response to Idha (when). As Abu Hayyan and others have stated, Idha is unique among conditional particles in that its response, when negated by in (not), la (not), or ma (not), does not require the fa connector, unlike other conditional particles.

His saying, (Is this the one whom Allah has sent as a messenger?) is the content of an implied speech; that is, they say, "Is this..." The sentence is in the position of a state (hal) pertaining to the subject of "they take you," or it is an incipient sentence in response to "What do they say?" It is also permitted that it be the [actual] response, with the sentence "(they take you...)" being parenthetical. The speaker of this [statement] was Abu Jahl and those with him; it is narrated that the verse was revealed concerning him.

The demonstrative pronoun is used here to express contempt, as in [the expression], "How strange it is regarding the son of Umar!" The pronoun referring back to the relative noun (al-ladhi) is omitted, meaning: "whom He has sent [him]." Rasulan (as a messenger) is a state (hal) derived from it, and it has the meaning of mursal (one sent). Abu al-Baqa permitted it to be an infinitive from which the possessor has been omitted—meaning: "possessor of a message"—but this is an unnecessary affectation.

Their presenting the fact that Allah—Exalted is He—sent him, may Allah bless him and grant him peace, as a messenger by making it the relative clause while they are in the utmost state of denial, is a form of sarcasm and mockery. Otherwise, they would have said, "Has Allah sent this one as a messenger?" It has been said that this is based on the estimation: "Is this the one whom Allah has sent as a messenger, in his own claim?" However, what preceded [the previous explanation] is more consistent with the state of those disbelievers, as well as being free from [the need for] estimation.