Tafsir of Az-Zumar 39:55

(And follow the best of what has been revealed to you from your Lord)

The apparent meaning is that this is an address to the servants who were addressed in the preceding verses, whether those intended are the believers or a group encompassing both them and the disbelievers. That which has been revealed refers to the Qur’an, and just as it was revealed to the believers, it was revealed to the disbelievers, given that it was revealed upon the Prophet (peace be upon him) to invite all of humanity.

The "best" of it refers to that which contains guidance toward the goodness of both abodes—not the historical accounts and the like—or it refers to the commandments, the categorical duties, or the abrogating verses. The superlative form (af‘al) in the first and third interpretations is literal, while in the second and fourth, there are two possibilities. It has been said: Perhaps the "best" is that which is most salvific and safest, such as turning [to Allah in repentance] and consistency in obedience, with the superlative form here also being literal.

It is permitted that the address is to mankind and jinn, and that "what has been revealed" refers to the heavenly scriptures, while "the best of it" refers to the Qur’an; however, this involves departing from the apparent meaning. Mentioning the Lord serves as an incitement to follow.

(Before the punishment comes to you suddenly)

Meaning: unexpectedly.

(And you do not perceive)

(55) You have absolutely no knowledge of its arrival so that you might rectify what would avert it.