Not to forgive means to allow this "fallen world" (Genesis 3:17), which we have to deny (1. John 2:15) to overpower you, what makes people to a part of this "lost world" (2. Peter 3:10), during to forgive with God's help means to overcome this evil world with their leaders, the rulers of darkness (Ephesians 6:12) and to be a part of God's Kingdom, which is not from this world (John 18:36).