What are strongholds as found in the Bible?
II Samuel 5:7 Nevertheless David took the strong hold of Zion: the same is the city of David.
The strongholds mentioned in the old testament were fortresses manned by the enemies of Israel which God commanded that they should conquer and destroy the inhabitants.
However, there is another kind of stronghold found in scripture. It is a stronghold that Christian believers are likewise to conquer and destroy. That kind of stronghold is based upon I Corinthians 10: 4-5.
II Corinthians 10:4 For the weapons of our warfare are not carnal, but mighty through God to the pulling down of strong holds;
5 Casting down imaginations, and every high thing that exalteth itself against the knowledge of God, and bringing into captivity every thought to the obedience of Christ
This kind of stronghold has to do with ideas, beliefs, desires, motivations, and sinful behaviors. It is manned by the devils of hell who managed to instill lusts, heresies, worldly attitudes, addictive habits, evil inclinations, and/ an inability to control ones emotions or affections and even compulsions and obsessions. I have counseled people with such problems in everything from lesbianism to thievery; from gossip to abortion; from sexual lusts to outright heresy and a lack of surrender to the will of God. Some of them I was able to help and others, the strongholds were so deeply implanted in the personality that they could not be helped because they didn't have the will to part company with their desires.
It is in the imagination (mind) that these strongholds are built by the devils. They lure believers into sinful activities or wrong beliefs, whichever be the case; they persuade the believer to practice the sin...in which every sin is like another brick in the walls of that stronghold. Evil spirits within these strongholds exist in the mind like cockroaches, or rats in a castle or perhaps like bats in an attic. They constantly seek to get control of the sinning persons choices and decisions in order to manifest through the personality, literally living through the persons evils and wrongdoings.
The personalities who man those strongholds are of course, devils who get their light and dark power from Satan just as the angels get their light and power from the Lord. They are defeated and brought down when a believer is willing to recognize the sin, admit it is a problem, confess the sin before God with true sorrow, and then forsake that sin forever. If this is done like it should be then the believer can get victory and be free from the bondage that sin brings. If the One who brought salvation to begin with is believed, trusted, and obeyed by the sinning one who allowed the sinful strongholds to be built in his mind and imagination then both victory and relief from oppression is given by the Lord.
Such a person is not 'possessed' in the classical understanding of the word but the stronger the evil spirits get over a persons will as it concerns their sin the more likely they are to manifest...........undetected in the personality. When a persons personality is eclipsed completely the person is then usually referred to as 'possessed'.
But then the obvious question arises: Can Christians indwelt by the Holy Spirit be invaded by demonic spirits and can their minds be controlled by them? Answer: If unconfessed sin is present, yes. But can they be possessed by the devil and owned by him? No. Possession involves ownership and every believer in Christ Jesus is a child of God. BUT.......believers can have their minds and imagination invaded by devils who can and do bring evil thoughts, aspirations, sinful desires, lustful longings, and a host of other sinful attitudes such as jealousy, rage, sinful anger, resentment, envy, gossip, lies, hate, and heresies. The proof that believers can be invaded by devils and even Satan himself is found in Acts 5 with Annanias and Sephira (Acts 5: 1-3). If a believer involves himself/herself in sin and won't part company with that sin then the devils can and will build a stronghold in their minds.
If this is not true then why would the apostle Paul even bother warning us about strongholds in the 'imagination' (vs 5)?
What I have told you above is the reason why the Christian world is so messed up, worldly, carnal, and there is so much dishonor to the One who redeemed us from sin by dying upon the cross.
The secret to victory over such things is not really a secret:
"If we confess our sins he is faithful and just to forgive us of our sins and the blood of Jesus Christ his son cleanseth us from all unrighteousness." I John 1:9. God gives us a lighted path through this dark world with victory, joy, happiness, and peace but only if we come clean from the sins that come between us and Him.
But we must be willing to part with those sins and eliminate the cause of it in our lives whether it is adultery, fornication, lies, theft, sexual lusts, or addictions of any kind.