Tafsir of An-Nahl 16:19

(And Allah knows what you conceal), meaning what you harbor within yourselves of beliefs and deeds, (and what you declare), meaning what you manifest of them. The pronoun referring back to the object is omitted [in the Arabic text] to observe the end-of-verse rhyme scheme, meaning: both matters are equal in relation to His knowledge—glory be to the All-Encompassing.

In placing the first before the second, there is an affirmation of equality in the most eloquent manner. In this, there is a threat and a manifestation of His—the Exalted—exclusivity to the attributes of divinity that is not hidden. As for the former, it is because the knowledge of the Sovereign, the All-Powerful, regarding the transgression of His servant necessitates His retribution, and frequently, when Allah’s knowledge and power are mentioned, that [retribution] is intended. As for the latter, it is based on what has been said: that the placement of the subject [in this context] signifies restriction. Hence, it has been said: He—glory be to Him—nullified their associating partners with the idols first by His saying—the Exalted: (Is then He Who creates like him who creates not?), and He nullified it a second time by His saying—blessed be His name: (And Allah knows...), as if it were said: He—the Exalted—is knowledgeable of this, unlike those you associate with Him, for they do not know this, nay, they do not know anything at all; so how can they be considered partners to the Knower of secrets and hidden things?

It is mentioned in Al-Kashf that the first sentence serves as an intimation that Allah did not task them with the due right of gratitude because it is impossible, and that He—glory be to Him—bypassed what is possible [to demand] in favor of what is easy and attainable. The second sentence intimates that they fell short even in this easy task, and thus became deserving of rebuke.