Jesus already had authority over satan and his works as demonstrated when he came in the flesh and walked upon this earth. Jesus is the perfect example of what we have available to us being joint heirs with him. What Jesus did for us at the cross as we recieved him was he brought us into a sonship relationship with God. We were seperated from God and were under the power of satan. By the mercy and grace of God we now have fellowship with God and authority over the enemy.