The root of bitterness lies with life events that we do not give to God. Disappointments that were never resolved or dropped can create chasms of darkness within the light. Death of a loved one or dissolution of a marriage can mar the faith of a person, and get covered over by habitual Christian life, and forgotten until the issue of trust in God is awakened again.
In order to get along with each other, we need to work out our issues as Jesus commands to. A healthy Christian is one whose attachment to worldly things has loosened and whose self-accountability for loving others is heightened. In terms of love, cynicism is a poison, and a record of wrongs is the undoing of the interconnectedness. When we belong to Jesus, seeking forgiveness is as regular as breathing, and if we know our former selves, we can forgive anyone of anything.
Relationships are between at least two people. If one of the two does not communicate that there is a problem, it festers and grows and the relationship fails. It is a sign that there is something wrong when someone talks to everyone else about a problem that they have with an individual. In the cases that I know of, where bitterness grew and people left the church, the people who communicated the least were the estranged parties.
If we have a problem with a pastor or the board, it is our responsibility as Christians to go to the party and tell them our problem in a loving way, and ask forgiveness for our part in it. We are always guilty when we assume someone has done us wrong on purpose. Only God knows the minds and hearts of anyone, and we are making assumptions when we blame anyone for attitudes or intentions that they may or may not have had.
So if you hold on to old resentments and disappointments, instead of surrendering them to God, and if you take offense and gossip to others instead of working it out with the person who offended you, you have offended Jesus, and even though he will never leave you, you will be without peace or joy until this is resolved. Let us resolve to deal with any differences between us, and any perceived wrongs with love and kindness, and to seek forgiveness for ourselves before we blame others for our problems.
Ephesians 4:31 Let all bitterness, and wrath, and anger, and clamor, and railing, be put away from you, with all malice.
Hebrews 12:15 looking carefully lest there be any man that falls short of the grace of God; lest any root of bitterness springing up trouble you, and thereby the many be defiled.