Marianne Williamson once said, "Unforgiveness is like drinking poison yourself and waiting for the other person to die."
There's nothing more toxic than the poison to the heart that comes through a grudge, and no other poison could be subtler. Toxicity of the heart is far more dangerous in that it doesn't just kill you slowly -- it also kills the people around you.
If there is one sure way to kill strong and God-ordained relationships, it's harboring bitterness and unforgiveness. Most people think that unforgiveness is rooted in a self-preservation reflex that helps us stay away from relationships that harm us. While unforgiveness does help us avoid bad relationships, it does way more damage than that.
Unforgiveness at it's very core is a strong feeling against someone who has maligned you, with you thinking that redemption is unlikely to happen. While some offenses are forgivable, such as being late for a meeting by two minutes, there are some offenses that some people think are beyond forgiveness.
However, God teaches a completely different perspective on forgiveness. There are two points that God wants to make very clear: point one, every offense is the same no matter how small or big; and point two, there is no offense that is beyond forgiveness.
God's example of forgiveness paints the most beautiful picture for us. 1 John 1:9 says, "If we confess our sins, he is faithful and just and will forgive us our sins and purify us from all unrighteousness." Acts 3:19 adds to that, saying, "Repent, then, and turn to God, so that your sins may be wiped out, that times of refreshing may come from the Lord."
God is not just in the business of forgiving people, but in a more serious business of restoring broken and maligned offenders to the point of inside-out transformation. And as God has forgiven us, He calls us to release forgiveness.
Matthew 18:21-35 tells the parable Jesus shares of an unforgiving servant. In Matthew 18:34-35, Jesus says, "And in anger his master delivered him to the jailers, until he should pay all his debt. So also my heavenly Father will do to every one of you, if you do not forgive your brother from your heart."
It's not so much that God won't forgive the unforgiving, but that He cannot. There are some things that God cannot do (God cannot sin, contradict Himself, etc) and one of those things is that He cannot work in an unforgiving heart.
That is why holding a grudge is so dangerous and toxic -- because it simply reflects that we have not yet understood the magnitude of God's redemptive work. If we truly understand how God has forgiven us, we will easily forgive others as well.
That's not to say that we put ourselves on the spot by trusting offenders. I know that takes time, but releasing someone and releasing ourselves from the clutches of bitterness can happen when we submit ourselves to the enabling grace that comes through submitting to Jesus's work in our lives.