"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" (Hebrews 12:15).
Bitterness and a bitter spirit should never characterize a person who has experienced the saving grace of God through Jesus Christ. No matter how seriously one may have been wronged, if he has known God's forgiving grace for his own wrongdoings, he should manifest that same grace in his life toward others, even though they do not merit it (for neither did he merit God's forgiving grace himself).
Bitterness is a characteristic of the ungodly "Whose mouth is full of cursing and bitterness" (Romans 3:14). A Christian must never try to rationalize it as "righteous indignation" or to think that certain injustices give him the "right" to be bitter and resentful. "If ye have bitter envying and strife in your hearts, glory not, and lie not against the truth. This wisdom descendeth not from above, but is earthly, sensual, devilish" (James 3:14-15). Rooted bitterness will soon "spring up," not only robbing the bitter believer of joy and true fruitfulness, but bearing bitter fruit whereby many others will "be defiled."
The antidote, of course, is never to "fail of the grace of God." That is, we need to be "looking diligently," moment by moment, at the wonderful grace of God by which we were saved through faith and in which we continue to live each day.
Therefore, "Let all bitterness, and wrath, and anger, and clamor, 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" (Ephesians 4:31-32). Otherwise, we not only hurt both ourselves and others, but we also grieve "the Holy Spirit of God" (v.30). HMM