Tafsir of Al-Mu'minoon 23:59

They dedicate worship solely to Him, the Almighty and Majestic. The intent is the negation of hidden shirk (polytheism), such as showing off in worship; this is what has been said. Some researchers have chosen to generalize the meaning, such that they do not associate with Him—the Exalted—any shirk, neither manifest nor hidden. This is perhaps the more appropriate view, and their previous description of believing in the signs of Allah does not render this redundant.

It is also possible that the preceding passage describes their affirmation of the Oneness of Lordship (Tawhid al-Rububiyyah), while this passage describes their affirmation of the Oneness of Divinity (Tawhid al-Uluhiyyah). The former was not deemed sufficient because most of the disbelievers are characterized by the affirmation of Lordship—as in: "And if you asked them, 'Who created the heavens and the earth?' they would surely say, 'Allah'" (31:25).

The inclusion of the title "Lord" (in the preceding verses) does not contradict this, for in all three instances, it serves to indicate causality. That title is suitable to serve as the reason for the Oneness of Divinity, as is not hidden.