My response: WRONG!!
I can see a good thought buried in the ideas presented, but there's more wrong than right with this definition!
This only works if the 'sin' was merely a personal quarrel (that was not spread or used in a larger group of people.) In that case, giving up the hard feelings and forgiving each other from the heart will indeed result in never using such past events against each other.
Even in this limited case - If the quarrel WAS spread to others (and didn't need to be), then you sinned against those people too by inciting them to 'take up a reproach' against your neighbor, and magnified the harm to your neighbor.* The only way to undo the damage - after repenting and forgiving each other - is to go back to the other people you (both?) brought into it, and repent of leading them into sin against your neighbor. If you posted ugliness publicly, then you need to repent just as publicly. Yes, you still need to set this straight even if you think you were 'right.' It probably won't fix everything, but this would be the minimum before you have seriously tried to rectify your sins before God and the wronged neighbor. If you don't, then your slander will go on, and your brother will find it a lot harder to leave the quarrel in the past when its still biting him in the..uh...backside.
* This includes mocking and contemptuous statements.
Followers of God are not to act like this!
3He does not slander with his tongue,
Nor does evil to his neighbor,
Nor takes up a reproach against his friend;
4In whose eyes a reprobate is despised,
But who honors those who fear the LORD;
He swears to his own hurt and does not change;
5He does not put out his money at interest,
Nor does he take a bribe against the innocent.
He who does these things will never be shaken. Psalm 15 NASB
There are real consequences those who have wronged others still need to face. You can (and actually need to) forgive those who actively attack you and your family, but you would be unwise to let them off the hook for the consequences (unless you are sure they have repented and you can afford to do so), and since that is 'using their past sins against them' - that is why I can't agree with this picture/sentiment.
If you figure the saying means that means you don't say the ugly about them beyond the necessary fact repetition to obtain (legal) recompense or defend yourself, then I can go with that...
Forgiveness is the decision to release the active resentment you feel toward those who have wronged you and yours. Its the sense that we are 'owed' something by someone else - and are willing to let it go. It doesn't come to us easily. Generally, we have to practice forgiveness - letting go of the same resentment, anger, pain, and shame we felt when it happened - over and over- before we are able to truly forgive others - or even accept forgiveness for ourselves.
Generally we desire vengeance AND restitution from the trespasser AND feel a sense of hostility toward the one who wronged us. It is HUGE part of humanity's makeup and is at the heart of what we think of as 'justice.' The desire for forgiveness is just as old.
In the ancient times, laws were enacted to satisfy people's innate desire for revenge and repayment.
Even the Biblical version called for "an eye for an eye...tooth for a tooth." Thieves might be required to pay up to 7x what they stole. Families were allowed to chase down and kill those who they had evidence had killed their loved ones, even in a proven case of manslaughter - if the person was outside certain designated areas of protection. These were considered reasonable penalties for wrongdoing.
Note that the heart was not addressed by these rules. People could get their restitution and go right on hating. They were just forbidden to act on it further. Hopefully those wronged would be satisfied with the restitution available. If not, too bad. Any actions beyond the legal remedies risked serious consequences to the unforgiving person.
The New Testament is a little different. Christ has paid the debt for all those who will accept his atonement, and we are not to seek our own revenge. God says vengeance is His and HE will repay.
Mind you, he said that in the Old Testament too, and people were still expected to pay up on their bills and lose a tooth if they knocked a tooth out of their neighbor's head.
The difference is -that in the New Testament, the call to forgiveness looks at our hearts first.
Can we 'let go' of the anger and the desire that the other should 'pay' -perhaps forever, for the wrong we believe they did? Will we adopt hatred and make it a way of life, feeding a demonic appetite to destroy/ruin the other person (and possibly their friends and families?) That is the path of UN-forgiveness. That is where it goes. Straight to hell.
I know of a woman who has attacked innocent people (and their families) with her mouth for decades over a wrong someone did her family. She lost one family member and thinks that all those anyone has accused of it should have hellish lives, be badly treated always and die- and their kids/relatives should be bad-mouthed and mistreated - and any friends should be chased away or mistreated. Her hatred has no end or proper limits. She doesn't see that she has become as bad or worse than the one who wronged her family. She seeks her own vengeance and attacks these innocent scapegoats, because she doesn't know who really did it, and she is determined that someone will pay. Its an example any real Christian would avoid.
I think this is why some serious believers have been encouraged to make a public statement of forgiveness to those they know did them wrong - one mother publicly forgave the drunk that killed her son this week. It was hard for her, but you can tell she meant it. With her forgiveness, the drunkard may eventually be able to accept Christ's forgiveness too. I think the Holy Spirit is trying to witness to those lost in unforgiveness - to realize how far gone they are and choose the better path.
For a Christian, Forgiveness is the decision to recognize that Christ's atonement paid for your sins and other people's sins toward you. Forgiveness is the decision to trust God to avenge you. You trust God to heal you and even make all things work together for the good as you trust in Him.
This is the wisdom of the doves. Forgiveness does not mean that you have to act like it never happened. Remembering who trod on your tail, and who would do so again in a heartbeat - that is the wisdom of serpents. We are not forbidden to use our brains and we are still allowed to defend ourselves.
"I am sending you out like sheep among wolves. Therefore be as shrewd as snakes and as innocent as doves." Mathew 10:16 NIV
If someone attacks my standing in a group of friends - especially with lies and prejudice, then they owe me an apology, at the least. Our relationship will not recover at all until they at least try to fix it.
Even if they never repent, I am called to forgive them - but in practice this will only mean I am not to seeking my own vengeance on them and am not sitting around hating their guts. Our relationship remains damaged. Trust is damaged. My respect for their character has taken a mortal blow. They are forgiven, but - in a very real sense - there will be lingering consequences. I am not obliged to treat someone who has acted like an enemy as if they were a friend. If I defend myself from the slander and refuse to speak with someone who only has my harm at heart, I am not failing as a Christian. God knows my heart.
You see, I am very serious about living my beliefs, so I take the implications of such definitions very seriously. This false definition of forgiveness in that image is exactly the sort of thing abusers might like to hang on me to keep me from seeking justice. Not vengeance, but the sort of protection the laws allow for their mistreatment of me and my family. If my circumstances were to improve dramatically, I could see someone trying to use such a definition to try and declare their past transgressions as irrelevant and demand I accept their current protestations of goodwill at face value - 'as a Christian.'
My family has been up against a small cadre of very nasty rumor mongers, and while I recognize the need to forgive them (and that takes Jesus' help too!) - I also recognize the need to discourage them from continuing to sin against us. I have been doing all I can to accept God's help and see my enemies as HE wants me to do. I have reached the point where I can pray for them and mean it, but it doesn't mean I will just take their abuse and not act when there are ways to make it unwise for these enemies to continue... Some martyrs/saints have been called to that action as a special gift of grace, but not the whole church all the time, or else we would never be able to seriously defend ourselves.
FORGIVENESS, by itself, does not remove all earthly consequences, not even in our relationships. That is a special grace called MERCY. Because God is merciful to those who sincerely repent, and calls us to be merciful to each other (that we may receive mercy), its not odd to think they go together. They do. However, the fact that consequences are not always removed when we are truly sorry is a clue. You may be sorry you broke your friend's car, but the car is still busted. Mercy & Forgiveness are related, but not the same.
Blessed are the merciful: for they shall obtain mercy. Matthew 5:7 ASV
He has showed you, O man, what is good; and what does the LORD require of you, but to do justly, and to love mercy, and to walk humbly with your God? Micah 6:8 ASV