Failure to deal with the spirit of unforgiveness will result to bitterness which eventually becomes a deadly poison in one's heart. Here is what the Bible say's about this subject matter:
1. Colossians 3:13 - Bearing with one another, and forgiving one another, if anyone has a complaint against another; even as Christ forgave you, so you also must do.
2. Luke 17:3 - Take heed to yourselves. If your brother sins against you, rebuke him; and if he repents, forgive him.
3. Ephesians 4:32 - And be kind to one another, tenderhearted, forgiving one another, even as God in Christ forgave you.
4. Mark 11:25-26 - 25 “And whenever you stand praying, if you have anything against anyone, forgive him, that your Father in heaven may also forgive you your trespasses. 26 But if you do not forgive, neither will your Father in heaven forgive your trespasses.”