The point here is to command people to purify their souls and warn them against corrupting them, as in His words:

“Successful indeed is he who purifies himself.”

If the meaning were assumed to be, “Successful indeed is he whose soul Allah has purified,” this would contain neither a command nor a prohibition addressed to them, neither encouragement nor warning. When the Quran commands or prohibits, it does not speak merely of “divine decree.” It does not say, “Whomever Allah has made a believer.” Rather, it says:

“Successful indeed are the believers.”

“Successful indeed is he who purifies himself.”

Mentioning divine decree alone in this context would defeat the intended purpose. Such speech would be unworthy even of someone with the weakest intellect—how, then, could it befit the speech of Allah? Do you not see that, when commanding, prohibiting, encouraging, and warning, He mentions the corresponding promises and threats, praise and blame? He mentions divine decree only when explaining His blessings upon them, whether blessings unrelated to their own actions or His blessing of faith and righteous deeds. He also mentions it when speaking of His power and will. In the context of commands, however, He mentions it only in connection with His blessings, as in His words:

“Were it not for Allah’s grace and mercy upon you, there would have been no purification…”

—and the rest of the verse. This is appropriate to that context.

His words, “Successful indeed is he who purifies himself,” and the verse under discussion belong to the second category, not the first.

The point is to discuss purification. Allah Most High says:

“Tell the believers to lower…”

—and the rest of the verse. He also says:

“Then go back; that is purer for you.”

And:

“Those who do not give zakah.”

And:

“You are not responsible if he does not purify himself.”

The basic meaning of “zakah” is an increase in goodness. Hence one speaks of the growth of crops and of the growth of