"Let all bitterness be put away from you with all malice" (Eph. 4:31).
Having been a Pastor for 44 years, I have seen firsthand the awful results of bitterness in a person's life. You only have to be around such a person for a few minutes to detect it. Like the Apostle Peter said to Simon the Sorcerer: "I perceive that thou art in the gall of bitterness" (Acts 8:23), I have witnessed the effects of bitterness and what it does to a person's life who is eaten up by it.
It's always someone else's fault. Somebody somewhere somehow said something that offended them years ago, and therefore they haven't gone to church since. Their spiritual life is shriveled up, dry, and obsessed with an offense they just can't let go of. I would try to tell them it is ruining their life, their happiness, their joy, their peace--but to no avail. They almost seem to enjoy harboring such feelings of bitterness because it gives them a convenient EXCUSE for turning against God and the Church.
The fruit of their life is rotten because the root is poisonous. The writer of Hebrews refers to bitterness as a ROOT: "Looking diligently lest any man fail of the grace of God; lest any root of bitterness springing up trouble you, and thereby many be defiled". Notice here that bitterness is not only called a ROOT, but that this root problem has an awful effect on others--many be defiled. Their negative, critical, ugly spirit is communicated to all who will listen to them.
The only cure for bitterness is forgiveness. That's why the same passage that warns us to "let all bitterness be put away from you" (Eph. 4:31), also gives us the secret regarding HOW TO DO IT: "But be ye kind one to another, tenderhearted, forgiving one another, as God for Christ's sake has forgiven you" (Eph. 4:32). You will never be able to let go of your bitterness until you practice true Biblical forgiveness.
"I can't do it!" you say? Well, certainly not in yourself. Such forgiveness only comes to those who allow Christ to forgive through them. He is a forgiving God and specializes in forgiving others. Even as He was dying on the cross, instead of becoming bitter at such injustice, He proclaimed: "Father, forgive them for they know not what they do".
So, whatever offense has ruined your spiritual life, let it go. Get over it. Forgive, forget, and move on. You will be happier and everyone around you will be happier too.