There are Christians who believe that because they are saved that they cannot be influenced by demons and therefore they cannot have any strongholds in their lives. That simply is not true.
They cannot be possessed unless God allows this to happen because the Holy Spirit has entered their body and has claimed it. However being possessed is not the same as being influenced.
Lets look at the two words:
Influence: to cause someone to change a behavior, belief, or opinion, or to cause something to be changed
Possess: have complete power over someone and be manifested through their speech or actions
They are not same and we as Christians need to understand that these two concepts are completely different. Failure to understand that a demon can try to influence you and lead you away from God is a huge gap in the Armor that God has provided for you and will allow Satan to potentially win.
Think about it this way, you may be able to influence people in your life. The person you are trying to influence can always say no, but that does not mean the influence went away. However, you do not possess them in order to control their actions to do what you wanted them do to begin with.
Can the Holy Spirit ever leave us once it has entered? People are under the assumption that once they are saved, they can basically do what they want. That simply is no true. So while you may be currently saved, Satan and his minions can whisper things into your ear to cause you to turn from and reject God. This is why God has asked us to prepare ourselves through reading His scripture and to put on His Armor.
Ephesians 6:11-18 New International Version (NIV)
11 Put on the full armor of God, so that you can take your stand against the devil’s schemes. 12 For our struggle is not against flesh and blood, but against the rulers, against the authorities, against the powers of this dark world and against the spiritual forces of evil in the heavenly realms. 13 Therefore put on the full armor of God, so that when the day of evil comes, you may be able to stand your ground, and after you have done everything, to stand. 14 Stand firm then, with the belt of truth buckled around your waist, with the breastplate of righteousness in place, 15 and with your feet fitted with the readiness that comes from the gospel of peace. 16 In addition to all this, take up the shield of faith, with which you can extinguish all the flaming arrows of the evil one. 17 Take the helmet of salvation and the sword of the Spirit, which is the word of God.
18 And pray in the Spirit on all occasions with all kinds of prayers and requests. With this in mind, be alert and always keep on praying for all the Lord’s people.
If when you were saved, that was it and from that point forward you were saved and nothing could ever reverse that decision, then there would be no real reason to do the above Bible versus However, God has instructed us in how to prepare for the World in which Satan has current control. Think about that for a bit.