Voluntary fasting and wearing of sackcloth was a custom in ancient times indicating occasions of grief or anxiety. We see an example of this in King Ahab’s grieving over his sin of idolatry followed by a humble and contrite heart as he repented before God (1 Kings 21:25-29).
When the Ninevites repented, “God saw their works, that they turned….” We see that the phrase, “their works” included repentance which they had learned through the preaching of God’s Word by Jonah (Jonah 3:2-5). They were obedient to the teachings of God when they heard those teachings (Jonah 3:4; cf. Romans 10:17), “believed God” and demonstrated their repentance (Jonah 3:5).
Repentance is therefore a necessary work of God to be obeyed.
In today’s religious world, some contend that no works at all are involved in conversion. If that’s the case, then no repentance is required in order to be saved. This view is contrary to biblical testimony (cf. Luke 13:3,5; Luke 24:47; Acts 2:38; Acts 3:19; Acts 17:30; 2 Corinthians 7:10)
—Mike Riley, Gospel Snippets