The most difficult math problem in the universe, it turns out, is 70 x 7. Perhaps the hardest thing to do in the Christian life is to forgive someone who has hurt you, often badly. But Jesus says the alternative to forgiving one's enemies is hell.
One of the reasons this is hard for us is because we too often assume forgiving a trespasser means allowing an injustice to stand. This attitude betrays a defective eschatology. At our Lord's arrest (Matt. 26:47-54), Jesus told Peter to put his sword back into his sheath not because Jesus didn't believe in punishing evildoers (think Armageddon). Jesus told Peter he could have an armada of angelic warriors at his side (and one day he will). But judgment was not yet, and Peter wasn't judge.
That's the point.
When we forgive, we are confessing that vengeance is God's (Rom. 12:19). We don't need to exact justice from a fellow believer because justice has already fallen at the cross. We don't need to exact vengeance from an unbeliever because we know the sin against us will be judged in hell or, more hopefully, when the offender unites himself to the One who is "the propitiation for our sins, and not for ours only but also for the sins of the whole world" (1 Jn. 2:2).
A prisoner of war who forgives his captor or a terminated pastor who forgives a predatory congregation, these people are not overlooking sin. Nor are they saying that what happened is "okay" or that the relationships involved are back to "normal" (whatever that is). Instead they are confessing that judgment is coming and they can trust the One who will be seated on that throne.
You don't have to store up bitterness, and you don't have to find ways of retaliation for what's been done to you. You can trust a God who is just. If you won't forgive, if you refuse to rest in God's judgment without seeking to retaliate, it doesn't matter what your evangelistic tracts and prophecy charts say. When it comes to the gospel and the to the end times, you're just another liberal.