Bitterness is a sin that often will lay undetected, and hidden for years. This is why we rarely see bitterness in the young. Yet eventually, what is in the heart will bear fruit. Jerry Bridges, in his book The Pursuit of Holiness, gives an excellent description of bitterness and a warning against the sins in our heart that may not be so obvious.
Bitterness arises in our hearts when we do not trust in the sovereign rule of God in our lives. If ever anyone had a reason to be bitter it was Joseph. Sold by his jealous brothers into slavery, falsely accused by his master’s immoral wife, and forgotten by one he had helped in prison, Joseph never lost sight of the fact that God was in control of all that happened to him. In the end he was able to say to his brothers, “You intended to harm me, but God intended it for good to accomplish what is now being done, the saving of many lives” (Genesis 50:20).
We can become bitter against God or against other people. Asaph was bitter against God because he felt God was not giving him a fair shake in life (Psalm 73:21). Job was bitter because he felt God was not recognizing his righteousness and even came to the place where his attitude was described as, “It profits a man nothing when he tries to please God” (Job 34:9).
Bitterness toward people is the result of an unforgiving spirit. Someone has wronged us, either apparently or actually, and we refuse to forgive that person. We refuse to forgive because we will not recognize that God has forgiven us of far, far greater wrongs. We are like the servant who, having just been forgiven a debt of several million dollars, had a fellow servant thrown into prison over a debt of a few dollars (Matthew 18:21-35). (Jerry Bridges, The Pursuit of Holiness)