To me, repentance means a complete change in your way of thinking and actions which involves attaining a completely new perspective of past actions; these past actions which now are repulsive to you and no longer bring pleasure to you in any way/shape/form.
Webster's dictionary defines repentance as: the action or process of repenting especially for misdeeds or moral shortcomings.
We find here that man sees repentance as a process, but how does God view repentance? I also noted that Websters listed "death sentence" as a phrase that rhymes with repentance, perhaps someone else will see the correlation with this article's subject matter. I won't try and over-complicate this by over-explaining the concept of repentance but the story of Jonah comes to mind here.
In the book of Jonah Chapter 3 we find that Jonah finally makes his way to Nineveh and proclaims the word of God to its inhabitants and tells the people that they must repent or die. Jonah preached the message of repentance or death sentence. The people of Nineveh believed God and proclaimed a fast immediately and when the king heard of Jonah's preaching he, made a proclamation that every man and animal had to fast and sit in sackcloth and ashes, symbols of contrition. The King hoped that God would accept their pledge of repentance and change his proclamation of doom.
We find in Chapter 4 that Jonah was afraid that God would change His mind and not destroy Nineveh (Jonah wanted it destroyed because they were enemies of Israel), that's why Jonah refused to go and warn the people at first. These are Jonah's words in Chapter 4: (1 But it displeased Jonah exceedingly, and he was very angry. 2 And he prayed unto the Lord, and said, I pray thee, O Lord, was not this my saying, when I was yet in my country? Therefore I fled before unto Tarshish: for I knew that thou art a gracious God, and merciful, slow to anger, and of great kindness, and repentest thee of the evil. 3 Therefore now, O Lord, take, I beseech thee, my life from me; for it is better for me to die than to live.)
We find several relevant concepts here:
-sometimes we are not taught the word of God because our enemies want us to be destroyed [Hosea 4:6].
-Jonah is upset with God for being gracious, merciful, slow to anger and showing great kindness; he is upset with God for having all the traits which have benefited Israel all these years. Like most hippocrits Jonah wants special treatment from God [Jonah 4:2].
-sometimes we disobey God because we don't like God's plan [Jonah 3].
-God loves everyone even our enemies and doesn't want anyone destroyed [2 Peter 3:8-10].
-animals are esteemed by God as much as humans: animals should not be mistreated [Jonah 4].
-we also find that as we repent from our evil ways, that God repents from the evil that he has proclaimed will be done to us! which is amazing...
Jonah shows his hardness of heart when he states that he would rather die than to see Nineveh be spared, after which God chides him for it. Sadly, we all know people with that mentality don't we. They pray and hope that their enemies perish, while their enemies are praying for salvation with repentant hearts. We sometimes forget that God sees the heart, while we just see the opponent or opposition and we want to destroy the competition.
So is repentance a process or does it need to happen instantly? The truth is, that Nineveh was told that they had 40 days before destruction, but we don't know how long of a space if any we will be given before God chooses to destroy us; therefore I would recommend that you  acknowledge your sin and repent immediately of your sinful ways; accept Jesus Christ as your Lord and Savior, the free gift of God;  confess and believe that Jesus died of your sins and that God has raised him from the dead.
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