Rashi: The matter that He will do. Radak ? it is called Sodo, for it is not known until He informs via His servants, the Nevi'im.

Sanhedrin 89b: One who withholds his prophecy is lashed. Other Nevi'im can warn him - "Lo Ya'aseh Hashem... Ki Im Galah Sodo" (He tells others).

Radak: Since He revealed to them to warn you, and you see that it comes like they told you, you will know why the evil came. Why do you not repent from your evil ways, and be saved from the evil?

Malbim: Do not say that they did not repent because the matter was already decreed, and they thought that Teshuvah will not help. The Navi replies, even though Hashem speaks and decrees, He does not do evil to carry it out immediately, only after revealing to the Nevi'im and sending them to warn the nation to repent, and the decree will be canceled and not be actualized. Surely the Navi will do his mission and go to warn the nation!