I've often taught people that the Bible does not teach forgiveness without repentance. This is not only true in terms of our relationship with God, but it is also true concerning our relationship with others. However, what I mean by "forgiveness" needs to be made a point of clarification.
There actually seem to be two kinds of forgiveness in the Bible. There is one that is an expression of no ill will toward another. I believe this is the type exercised by Christ on the cross when He said, "Father, forgive them, for they know not what they do." This is the type of forgiveness that prays for the repentance of others. It seeks their good instead of calamity for them. It desires that they would turn and have life rather than continue in their rebellion toward God and toward death.
This type of forgiveness, I think, is what Christ taught us to have in terms of blessing those who curse us, praying for those who persecute us, loving those who hate us. This type of forgiveness should be given to all people everywhere by Christians as an enduring monument of Christ's love for us.
However, the type of forgiveness described in texts like Matthew 18 may have its foundation in the above attitude/forgiveness, but it is only granted when one repents of his offense. This is because forgiveness of this variety is a restoration of relationship. It isn't simply good-will toward another, or seeking their repentance, but a full-blown restoration of the offender born in the forgetting of the offense. This is also why church discipline is practiced. Church discipline is not a process of restoring a brother who has sinned, but has now repented, so that he can be acceptable again to the church. The goal of church discipline, other than its primary goal to remove a corrupting influence from other Christians who may fall because of the offender, is to bring the offender into a state of repentance, so that he can be forgiven (i.e., restored to communion with God and fellowship with the Church).
This is the main kind of forgiveness in the Bible, and I'm afraid that the two are often confused to our own detriment. If we confuse the former with the latter, we end up with a variety of unChristian practices. One of these is to communicate a cheap grace, where all offenses are just forgiven and there is no need to repent from them in order to receive restoration to God and one another. Another bad practice might be the opposite where we begin to wish ill-will toward others because they have not yet repented.
The Book of Jonah has much to say concerning a heart that does not want people to repent, but to pay for their crimes. Assyria in particular was one of the harshest in war. The author, no doubt, speaks to readers who would have absolutely hated them with good cause. It was part of what scholars call the "ideology of terror" in Assyrian campaigns to not only brutally kill, but desecrate the bodies in the most shameful of ways after the massacre had taken place. Many families were not merely torn apart figuratively. One can scarcely imagine what it would be like to hate a people who clearly deserved to be hated so much. We all are outraged at the current gunman who killed his victims in a theatre. I feel such indignation for him that, to speak the truth, I hardly want him to repent, but rather to pay for his crimes in hell. And these people who were killed were not even my family or friends. Imagine what a woman who had lost her husband and sons to a brutal attack by the Assyrians only to have them desecrate their bodies afterward. You can then see why Jonah in the story has such a hard time with it. People act like Jonah's just a jerk, but if we would have such a feeling toward a man who killed people we have never even met, then we should certainly be empathetic toward an Israelite who hated the brutal murderers of people he did know.
Now, this is where it gets tricky, because the Bible also relates to us that if people are not going to repent, we should desire their destruction. This is important. We are to actually hate evil people who will never turn. And we ought to pray to God for His justice to bring calamity upon the wicked who will not be repenting. But this is a generic prayer, as we never know who it is who will repent who will not. Hence, our prayers in general may condemn the wicked, but our specific prayers should seek the repentance of others so that they are not destroyed. Instead of being as Jonah on the mountain, getting angry with God for forgiving the Ninevites when they repented, we need to be rejoicing that what was dead is now alive, what was murderous is now life-giving.
In other words, what we are seeking when we forgive people with what we will call Forgiveness A in the Bible is their repentance, so that they can be delivered from the wrath of God. We are not seeking for God to ignore whether they repent and to save them anyway. That is not what Christ's prayer is seeking on the cross either, as He has already declared that many of those people will be damned if they do not repent. So in Forgiveness A, what we are really seeking then is that God would grant them repentance so that they might receive Forgiveness B (i.e. a restored, salvific relationship with Him).
I think in confusing all of this many Christians today think it's wrong to condemn the wicked in general, that they should forgive and forget and restore any relationship with a person regardless of repentance, or that they should seek the destruction of specific individuals since those individuals have not yet repented. All of this is very confusing if it is not understood to what situation the three paths address (i.e., unrepentant sinners in general who will not repent in the end, unrepentant sinners who may repent in the end, and repentant sinners).
So let us have this attitude in us, to always be ready to pray for those who have done evil so that evil might be destroyed by God's love and grace in the sinner's turning to Him in repentance, but if he does not, let us pray that evil will be destroyed by God's wrath and justice upon those who produce it. May He be glorified in the eradication of evil by both divine actions taken, and may we therefore exalt Him through both. But grudges that would wish destruction upon others rather than repentance is not of the Spirit of Christ anymore than restoring others to ourselves when they have not asked to be restored. In keeping with all that the Bible shows us, we end up preserving the full message of God that relates His victory over evil through both justice and love.
Matthew 18 is a great chapter for this, precisely, because it deals with all of the above (condemnation of those who would unrepentantly be stumbling blocks by sinning against others, the compassionate desire to seek one who is lost, the forgiveness of one who has repented, and the discipline that seeks for repentance when it is not immediately given. I can leave you with nothing greater then than the words of the Lord Jesus Christ.
At that time the disciples came to Jesus and said, "Who then is greatest in the kingdom of heaven?" And He called a child to Himself and set him before them, and said, "Truly I say to you, unless you are converted and become like children, you will not enter the kingdom of heaven. "Whoever then humbles himself as this child, he is the greatest in the kingdom of heaven. "And whoever receives one such child in My name receives Me; but whoever causes one of these little ones who believe in Me to stumble, it would be better for him to have a heavy millstone hung around his neck, and to be drowned in the depth of the sea.
"Woe to the world because of [its] stumbling blocks! For it is inevitable that stumbling blocks come; but woe to that man through whom the stumbling block comes! "If your hand or your foot causes you to stumble, cut it off and throw it from you; it is better for you to enter life crippled or lame, than to have two hands or two feet and be cast into the eternal fire. "If your eye causes you to stumble, pluck it out and throw it from you. It is better for you to enter life with one eye, than to have two eyes and be cast into the fiery hell. "See that you do not despise one of these little ones, for I say to you that their angels in heaven continually see the face of My Father who is in heaven.
["For the Son of Man has come to save that which was lost]. "What do you think? If any man has a hundred sheep, and one of them has gone astray, does he not leave the ninety-nine on the mountains and go and search for the one that is straying? "If it turns out that he finds it, truly I say to you, he rejoices over it more than over the ninety-nine which have not gone astray. "So it is not [the] will of your Father who is in heaven that one of these little ones perish.
"If your brother sins, go and show him his fault in private; if he listens to you, you have won your brother. "But if he does not listen [to you], take one or two more with you, so that by the mouth of two or three witnesses every fact may be confirmed. "If he refuses to listen to them, tell it to the church; and if he refuses to listen even to the church, let him be to you as a Gentile and a tax collector. "Truly I say to you, whatever you bind on earth shall have been bound in heaven; and whatever you loose on earth shall have been loosed in heaven. "Again I say to you, that if two of you agree on earth about anything that they may ask, it shall be done for them by My Father who is in heaven. "For where two or three have gathered together in My name, I am there in their midst."
Then Peter came and said to Him, "Lord, how often shall my brother sin against me and I forgive him? Up to seven times?" Jesus ^said to him, "I do not say to you, up to seven times, but up to seventy times seven. "For this reason the kingdom of heaven may be compared to a king who wished to settle accounts with his slaves. "When he had begun to settle [them], one who owed him ten thousand talents was brought to him. "But since he did not have [the means] to repay, his lord commanded him to be sold, along with his wife and children and all that he had, and repayment to be made. "So the slave fell [to the ground] and prostrated himself before him, saying, `Have patience with me and I will repay you everything.' "And the lord of that slave felt compassion and released him and forgave him the debt. "But that slave went out and found one of his fellow slaves who owed him a hundred denarii; and he seized him and [began] to choke [him], saying, `Pay back what you owe.' "So his fellow slave fell [to the ground] and [began] to plead with him, saying, `Have patience with me and I will repay you.' "But he was unwilling and went and threw him in prison until he should pay back what was owed. "So when his fellow slaves saw what had happened, they were deeply grieved and came and reported to their lord all that had happened. "Then summoning him, his lord ^said to him, `You wicked slave, I forgave you all that debt because you pleaded with me. `Should you not also have had mercy on your fellow slave, in the same way that I had mercy on you?' "And his lord, moved with anger, handed him over to the torturers until he should repay all that was owed him. "So will My heavenly Father do to you, if each of you does not forgive his brother from your heart."