God saw their works, that they turned from their evil way. God repented of the evil which he said he would do to them, and he didn't do it.
But sometimes the prophetic revelation is an impressed likeness of Divine Foreknowledge, as it knows the order of causes to effects; and then at times the event is other than is foretold, and yet there is nothing untrue in the prophecy. For the meaning of the prophecy is, that the disposition of the inferior causes, whether in nature or in human acts, is such, that such an effect would follow” (as in regard to Hezekiah and Nineveh), “which order of the cause to the effect is sometimes hindered by other things supervening. “The will of God,” he says again, “being the first, universal Cause, does not exclude intermediate causes, by virtue of which certain effects are produced. And since all intermediate causes are not adequate to the power of the First Cause, there are many things in the power, knowledge, and will of God, which are not contained in the order of the inferior causes, as the resurrection of Lazarus. Whence one, looking to the inferior causes, might say, ‘Lazarus will not rise again:’ whereas, looking to the First Divine Cause, he could say, ‘Lazarus will rise again.’ And each of these God willeth, namely, that a thing should take place according to the inferior cause: which shall not take place, according to the superior cause, and conversely. So that God sometimes pronounces that a thing shall be, as far as it is contained in the order of inferior causes (as according to the disposition of nature or deserts), which yet doth not take place, because it is otherwise in the superior Divine Cause. As when He foretold Hezekiah Isaiah 38:1, “Set thy house in order, for thou, shalt die and not live;” which yet did not take place, because from eternity it was otherwise in the knowledge and will of God which is unchangeable. Whence Gregory saith , ‘though God changeth the thing, His counsel He doth not change.’ When then He saith, “I will repent,” Jeremiah 18:8. it is understood as said metaphorically, for men, when they fulfill not what they threatened, seem to repent.”