Lev 19:18 You shall not take vengeance, nor bear any grudge against the sons of your people, but you shall love your neighbor as yourself; I am the Lord.
Unforgiveness gives satan an opportunity
2 Cor 2:9-11 For to this end also I wrote, so that I might put you to the test, whether you are obedient in all things. 10 But one whom you forgive anything, I forgive also; for indeed what I have forgiven, if I have forgiven anything, I did it for your sakes in the presence of Christ, 11 so that no advantage would be taken of us by Satan, for we are not ignorant of his schemes.
Heb 12:15 See to it that no one comes short of the grace of God; that no root of bitterness springing up causes trouble, and by it many be defiled;
Unforgiveness breaks our fellowship with God