Most (independent Baptist) churches I visit are full of the gall of bitterness and most members of these don't realize that they have this problem until it is pointed out to them by the Holy Spirit. Bitterness does bad things to us and to our churches. It ensures that we will not let the Grace of God into our circumstances. Eve was bitter to God because He withheld the fruit.
One man pastored a southern church for 25 years before running off with the church secretary. His son said that first off he got bitter because the small, never growing church had treated him like dirt all those years and as a consequence he became bitter - and also immoral. Some kind of moral defilement follows bitterness.
The going down of the root of bitterness is followed always by the springing up of some other things. It can be like Spring Bamboo - after it is planted nothing happens for four years but in the 5th year it springs up suddenly to 90 feet high!
Furthermore, bitter people enjoy being bitter, a satisfaction in getting back at others. Bitterness is the grieving sin of Ephesians 4:31 which begins with God, ie. we feel we didn't get what we deserved of Him.
Let all bitterness, and wrath, and anger, and clamour, and evil speaking, be put away from you, with all malice: (Eph 4:31)
One pastor was very bitter because although Job at the end got double the goods God promised, he only got the same number of children - this pastor felt that this was proof that God could not be trusted to do what He said he would do.
Our daughter is handicapped and we ask why did God allow this to happen to us? Bitterness often begins with hatred to God and ends with an unforgiving spirit to others.
The case of the unforgiving man in Matthew who was forgiven but turned round and cast his own debtor into prison. The bitter waters of Mara were made sweet by casting in a tree and hence also the tree of Calvary. What was done to you might have been wrong but your response to that has also been wrong...
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.
(Peter, Ingham, Qld)