Jesus triumphed over Satan and all his demons through His work of redemption, particularly by canceling the power of sin at the cross. (Matt. 12:28-29; Eph. 1:19-23; Col. 2:13-15) Satan is already condemned. (John 12:31; 16:11; Rev. 20:7-10)
The principle of authority is that we are in the kingdom of Christ. Jesus is Lord and head of all power and authority. Satan has absolutely no legitimate authority over the believer. We are free from Satan’s claims upon us. All sins are forgiven, all condemnation is ended, all cultic vows, promises, covenants, etc., were broken when a person comes into Christ's sphere of authority (Col. 1:13-14; 2:9-15). One must never compromise this principle by saying that sin gives a demon legal right to invade a believer or attach itself to your soul. Believe in God; disbelieve Satan.
Our primary weapons against demonic stratagems are faithful application of the fundamentals of the faith (Eph. 6:10-20; 1 Pet. 5:6-9). Light expels darkness.