Hebrews 12:15 says: "See to it that no one fails to obtain the grace of God; that no “root of bitterness” springs up and causes trouble, and by it many become defiled..." (ESV) Where do bitter roots come from? A person gets hurt or offended or slighted. It may've been deliberate or unintentional. It may've been an offense that was perceived but not actual. Anger erupts. Maybe even towards God. But they do nothing. The hurt festers, the wound deepens, hurt morphs into resentment. Resentment decomposes into bitterness and the poison spreads like gangrene. Ephesians 4:26-27 is preventative medicine: "Be angry and do not sin; do not let the sun go down on your anger, and give no opportunity to the devil." (ESV) But 1 John 1:5-7 is the Gospel remedy for a bitter heart: "But if we walk in the light, as He is in the light, we have fellowship with one another, and the blood of Jesus His Son cleanses us from all sin. If we say we have no sin, we deceive ourselves, and the truth is not in us. If we confess our sins, He is faithful and just to forgive us our sins and to cleanse us from all unrighteousness." (ESV)