Tafsir of As-Saffat 37:4

(Indeed, your God is One)

This is the response to the oath. It has been their custom to emphasize what is of importance by preceding it with an oath, which is why it was placed first here. Therefore, it cannot be said: "This is a statement directed toward one who denies and disbelieves, so there is no benefit in the oath."

Furthermore, what has been stated—that the oneness of the Creator is established by transmitted proof after being established by reason, and thus its benefit is apparent here—is not entirely complete, because the discourse is with those who do not acknowledge monotheism. The proof has been alluded to in His, the Glorified's, statement: