Deuteronomy 29:18 (AMP)
Beware lest there should be among you a man or woman, or family or tribe, whose [mind and] heart turns away this day from the Lord our God to go and serve the gods of these nations; lest there should be among you a [poisonous] root that bears gall and wormwood.

According to the Webster dictionary, bitterness is the quality or state of being bitter, sharp, or acrid, in either a literal or figurative sense, grief, implacableness; resentfulness; severity; keenness of reproach or sarcasm; deep distress, or vexation of mind and spirit.
Our world is thriving on so much bitterness right now that it must grieve the Lord to see so much inhumanity to man that stems out of it. Bitterness is everywhere you turn, even in the church. Believers are full of bitterness like Simon in Acts 8:21-24 because of their unrepentant nature which does not get the answers to their prayers.
A root that bears gall and wormwood probably refers to an evil heart of unbelief that has departed from the living God and could also signify false doctrines, or idolatrous persons among genuine believers.
Have you ever experienced that sense of anguish of the soul that brings bitterness either from unfulfilled desires or from frustration? Job 7:11 tells us he got to that point, where he saw all was hopeless and decided to indulge himself in complaining about his circumstances.
Life can be so frustrating, and we all experience frustration. It usually comes from trying to do something about a situation we cannot do anything about. We get frustrated because it seems things are taking too long, or no one is willing to help us. We get frustrated because the money we need is not coming in, or the aches, pains, and burdens we have prayed to be removed from us keep going on and on. During his trials and difficulties, Job felt as though there was no one to come to his aid or defence in Job 9:32, 33. Have you ever felt this way?
A root of bitterness signifies a poisonous plant. The root of bitterness is used in the bible metaphorically for bad people, or a man holding unsound doctrines, and craftily spreading them in the Church.
Poisons are not good things, more so to the soul and the spirit, every root of bitterness is a poison to man’s soul and creates all kinds of trouble.
Hebrews 12:15 cautioned us to avoid every work of bitterness, especially bitterness that comes from intense animosity and resentment, referring to bitter roots bearing bitter roots such as bearing ill-will towards someone, and unresolved anger.
We must not have a sinful and unbelieving heart according to Hebrews 3:12, this creates a bitterness that causes a turning away from the Lord and rejecting faith. We’ve met people who left church or “God” out of bitterness because of their selfishness, and ignorance in not getting their way.
Epiphanes 4:31 says that Christians should not indulge in bitterness of spirit. I have known people who say they are Christians but do much evil in a Christian society out of the bitterness of spirit and soul.
Romans 3:14 says some’s mouth is full of cursing and bitterness. These people never speak anything good but in profane oaths, blasphemies, and malice.
Bitterness has no place in the life of a believer and we should not allow the root of bitterness to poison our lives or others.
Shalom.