Tafsir of Az-Zumar 39:18

(Those who listen to the word and follow the best of it): This is a commendation of them, as they are critics in religion; they distinguish between the good and the best, and the virtuous and the most virtuous. Thus, if two matters confront them, such as an obligatory act and a recommended one, they choose the obligatory; likewise, with a permissible act and a recommended one.

It has been said that they listen to the word—or the command of Allah Almighty—and follow the best of it, such as retaliation versus forgiveness, seeking victory versus forbearance, and open giving versus secret giving, according to the words of Allah Almighty: “And to forgive is nearer to piety,” and “If you disclose your Sadaqat (alms-giving), that is well, but if you conceal them and give them to the poor, that is better for you.” The difference between the two interpretations is that the latter is more specific, as it pertains to commands where there is a choice between a superior option and its alternative (such as forgiveness versus retaliation). It is as if it is said: they follow the better of two specific rulings. In the first interpretation, they follow the better of any two statements, such as the obligatory relative to the recommended.

According to Al-Zajjaj, they listen to the Quran and other things, but follow the Quran. It is also said that they listen to the word from whomever it comes, then follow that which is most worthy of acceptance and most guided toward the truth. It is implied by this description that they distinguish the ugly from the beautiful and avoid the ugly. Those intended here are the servants who have avoided [idolatry] and turned in repentance, and no others, so that the order of the composition does not fall apart; for the words of Allah Almighty, “So give glad tidings,” are consequent to His saying, “For them are glad tidings.” The explicit noun was used in place of the pronoun to honor them through the attribution [of the title "My servants"] to Himself, to reiterate the proof of their merit, and to indicate that they are discerning critics, eager to prefer obedience and attain greater proximity to Allah Almighty; this fulfills the [nature of] repentance and perfects its excellence.

It is said that the pause is at “‘ibadi” (My servants), in which case “alladhina” (those who) is the subject, and the predicate is the sentence of His saying, “Those are the ones whom Allah has guided”—that is, to His religion—and the discourse is an inception, repeating the description of those whose narrative was initiated. The preceding view is weightier due to the benefits already mentioned, such as placing the explicit noun in place of the pronoun and the completion of the meaning, for that would not be complete without the description. Furthermore, the impetus for the question answered by the sentence following His saying, “and follow the best of it,” is stronger, and that is the principle for an excellent inception.

(And those are the people of understanding): That is, they are the owners of intellects free from the opposition of illusion and the disputes of desire, who are worthy of guidance and no others. The verse contains an indication of the low status of pure imitation (Taqlid). Thus it has been said: "Exert yourself and be a diligent seeker (mujtahid) in matters of religion, and do not be like a donkey tied [to a post] that is led." It is also inferred from this that guidance is achieved through the action of Allah Almighty and the soul’s acceptance of it, as held by the Ash’arites.