Strive for peace with everyone, and for the holiness without which no one will see the Lord. 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 - Hebrews 12:14-15
What a "root of bitterness"? The first link is a list of Bible verses talking about bitterness and the second link is an explanation about what a root of bitterness is:
