This teaching comes from 2 Corinthians 2:10–11 (RSV), where Paul urges the Corinthian church to forgive and comfort a repentant offender. He explains that forgiveness is important “so that we would not be outwitted by Satan; for we are not ignorant of his designs.”

Paul’s point is that unforgiveness gives Satan a foothold to cause division, bitterness and strife within the Christian community.

2 Corinthians 2:10–11 (RSV)
“Any one whom you forgive, I also forgive. What I have forgiven, if I have forgiven anything, has been for your sake in the presence of Christ, to keep Satan from gaining the advantage over us; for we are not ignorant of his designs.”