And if you obey a human being like yourselves

"And if you obey a human being like yourselves"—in what has been mentioned of states and attributes—meaning: if you comply with his commands, "then you would indeed be losers"—in your intellects and cheated in your opinions, for you have debased yourselves.

The lam (in la-khasirun) serves to introduce an oath. The sentence "you would indeed be losers" is the response to the oath. As for idha (then), in my view, it is adverbial, relating to the affirmation implied by the predication between the subject and the predicate, or relating to the predicate itself; the lam does not prevent it from governing in such a construction. The response to the conditional clause is elided, indicated by the aforementioned statement.

Abu Hayyan stated: "If this were the response to the conditional, the fa (particle) would be mandatory, such that it would be said: fa-innakum (for then you would be...). Moreover, even if it were with the fa in a construction other than the Noble Quran, that construction would not be permissible except according to al-Farra'; the Basrans do not permit it, and it is considered an error by them."

Some have mentioned that idha here functions as the recompense (jaza') and the response, and they have labored to justify this, though there is no call for such exertion other than the supposition that it is mandatory to follow the well-known position and that the truth regarding adherence in these contexts is limited to what the majority hold. In Ham' al-Hawami'—and likewise in al-Itqan by Jalal al-Suyuti—there is information in this discussion that will benefit you, so refer to it.