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 if any one objects and says that still this view does not prevent us from thinking that good works reconcile us to God, and that they thus procure our salvation: to this I answer — that the question here is not about the procuring cause of forgiveness. It is certain that God was freely pacified towards the Ninevites, as he freely restores his favor daily to us. Jonah then did not mean that satisfactions availed before God, as though the Ninevites made compensations for their former sins. The words mean no such thing; but he shows it as a fact which followed, that God was pacified, because the Ninevites repented. But we are to learn from other parts of Scripture how God becomes gracious to us, and how we obtain pardon with him, and whether this comes to us for our merits and repentance or whether God himself forgives us freely. Since the whole Scripture testifies that pardon is gratuitously given us, and that God cannot be otherwise propitious to us than by not imputing sins, there is no need, with regard to the present passage, anxiously to inquire why God looked on the works of the Ninevites, so as not to destroy them: for this is said merely as a consequence. Jonah then does not here point out the cause, but only declares that God was pacified towards the Ninevites, as soon as they repented. But we shall speak more on this subject. 