Unforgiveness: Satan’s Advantage Against Us

For to this end also did I write, that I might know the proof of you, whether ye be obedient in all things.  To whom ye forgive any thing, I forgive also: for if I forgave any thing, to whom I forgave it, for your sakes forgave I it in the person of Christ; lest Satan should get an advantage of us: for we are not ignorant of his devices.
2 Corinthians 2:9-11