Definition of Bitterness : Holding onto or showing feelings of intense animosity, resentment or vindictiveness. Other words that describe it are : merciless, unforgiving, holding a grudge. Bitterness is also described as feelings resulting from something that is difficult to accept.
"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;"(He 12:15)
"Repent therefore of this thy wickedness, and pray God, if perhaps the thought of thine heart may be forgiven thee. For I perceive that thou art in the gall of bitterness, and in the bond of iniquity."(Acts 8:22-23)
"Husbands, love your wives, and be not bitter against them."(Col 3:19)
"But if ye have bitter envying and strife in your hearts, glory not, and lie not against the truth."(James 3:14)
"Thou shalt not avenge, nor bear any grudge against the children of thy people, but thou shalt love thy neighbour as thyself: I am the LORD."(Lev 19:18)
"Let all bitterness, and wrath, and anger, and clamour, and evil speaking, be put away from you, with all malice: And be ye kind one to another, tenderhearted, forgiving one another, even as God for Christ's sake hath forgiven you."(Eph 4:31-32)
"For if ye forgive men their trespasses, your heavenly Father will also forgive you: But if ye forgive not men their trespasses, neither will your Father forgive your trespasses."(Mt 6:14-15)
In His parable of the unmerciful servant, Jesus made it clear that we will suffer if we refuse to sincerely forgive our brethren. "So likewise shall my heavenly Father do also unto you, if ye from your hearts forgive not every one his brother their trespasses."(Mt18:35)
"And above all things have fervent charity among yourselves: for charity shall cover the multitude of sins."(1Pe 4:8)
"Therefore I will not refrain my mouth; I will speak in the anguish of my spirit; I will complain in the bitterness of my soul."(Job 7:11)
"Behold, for peace I had great bitterness: but thou hast in love to my soul delivered it from the pit of corruption: for thou hast cast all my sins behind thy back."(Isa 38:17)
" let not the sun go down upon your wrath: Neither give place to the devil."(Eph 4:26-27)
"Therefore if thou bring thy gift to the altar, and there rememberest that thy brother hath ought against thee; Leave there thy gift before the altar, and go thy way; first be reconciled to thy brother, and then come and offer thy gift."(Mt 5:23-24)
"And when ye stand praying, forgive, if ye have ought against any: that your Father also which is in heaven may forgive you your trespasses."(Mk 11:25)
"Forbearing one another, and forgiving one another, if any man have a quarrel against any: even as Christ forgave you, so also do ye."(Col 3:13)
"Brethren, I count not myself to have apprehended: but this one thing I do, forgetting those things which are behind, and reaching forth unto those things which are before,"(Phil 3:13)
"And be not conformed to this world: but be ye transformed by the renewing of your mind, that ye may prove what is that good, and acceptable, and perfect, will of God."(Ro 12:2)
"And be renewed in the spirit of your mind;"(Eph 4:23)
"And we know that all things work together for good to them that love God, to them who are the called according to his purpose."(Ro 8:28)