Tafsir of Az-Zumar 39:13

(Say, "Indeed, I fear, if I should disobey my Lord") by abandoning sincerity and inclining toward that which you are upon of polytheism. It is also permissible to interpret this generally—meaning: "I fear if I disobey Him in any of the sins"—(the punishment of a tremendous Day).

(It is the Day of Resurrection.) Describing it as 'tremendous' refers to the magnitude of the calamities and terrors contained within it. This is a metaphor involving the temporal setting or the attribution [of the adjective], and it is more emphatic; hence, the description was diverted from the punishment to the Day itself. The intended purpose of saying this to them is to threaten them and to expose them, for if he—upon him be blessings and peace—despite his greatness, would not be secure from the punishment if he were to disobey Allah the Exalted, then how could they be?