Continuing from my previous entry about this drama...
When the chief of the local police (Park Chul-Min) first receives a bribe from the merchant Yang Man-Oh (Lee Chun-Hee), he has given Man-Oh a foothold by allowing him to smuggle in a ship load of illegal goods. However, as he continues to accept more bribes from Man-Oh, he relinquishes more and more ground to the merchant who can then keep smuggling in and selling off even more illegal goods.
At the same time, Man-Oh is sending bribes to other officials including some high-ranking ministers. Gradually, he builds up a stronghold where he has control over all those corrupted officials who have accepted his bribes. So much so that they give him permission to conduct all kinds of illegal activities under their noses.
Similarly, if we sin and give Satan an opening to cause a one-time disturbance then it is a foothold. However, if we continue to sin and allow him to gain increasingly more control over our life, then it becomes a stronghold.
If the police chief had come clean after receiving the first bribe and stopped accepting further bribes, Man-Oh would not have been able to gain control over him. By the time the merchant has extended his control over the many corrupted officials, they already find it extremely difficult to stop him and his illegal activities.
In the same way, we need to confess and repent of our sin that gives the devil a foothold and kick him out. Otherwise, if we neglect to do that and the enemy builds a stronghold in us (eg addictions or negative thought patterns), it would take much more effort to remove his control over us.
Read more about "Conspiracy in the Court" - Power vs Authority