Tafsir of At-Taghabun 64:13

(Allah, there is no deity except Him) The discourse regarding this is the same as the discourse regarding the word of Tawhid, which has already been discussed.

(And upon Allah) i.e., upon Him, Exalted is He, exclusively, without anyone else, neither independently nor by association, (let the believers rely). The use of the name of Majesty (Allah) in the position where a pronoun might have been expected serves to indicate the reason for reliance, or the reason for the command to rely; for divinity necessitates turning solely to Him, the Exalted, in entirety, and severing attachment completely from all other created beings.

Some of the eminent scholars have mentioned that the command to rely is restricted to the believers because the belief that all things are from Allah, the Exalted, necessitates reliance. Hence, it has been said: Among all the verses, there is none that urges reliance upon Allah, the Exalted, more powerfully than this verse, due to its implication that whoever does not rely upon Allah, the Exalted, is not a believer.

It is, as al-Tibi stated, like a conclusion and a summary of what has preceded, and a transition to another subject.