Neil Anderson, The Bondage Breaker, p. 181
Do we have blanket immunity from Satan's invasion? No, that protection is conditional on our responsible participation in God's plan for our protection. Dr. Unger writes: "The Holy Spirit indwelling the believer ungrieved by sin (Eph 4:30>) and unquenched by disobedience (1 Thes 5:19>) most certainly precludes invasion by a demon spirit. But who dares assert that a demon spirit will not invade the life of a believer in which the Holy Spirit has been grieved by serious and persistent sin and quenched by flagrant disobedience?"