After doing a study on Mark 5:1-20 where Jesus heals a man who was possessed by possibly thousands of demons, I pondered the question, "Can Christians become possessed?" I believe that the scriptures make it clear that the answer is no, Christians cannot become possessed. Ephesians 1:13-14 says this, "In him you also, when you heard the word of truth, the gospel of your salvation, and believed in him, were sealed with the promised Holy Spirit, who is the guarantee of our inheritance until we acquire possession of it, to the praise of his glory." When I think of the fact that we are "sealed" with the Holy Spirit, I think of a glass jar. Before you put the lid on the jar, contents can be added to the jar and removed from the jar. However, once you put on the lid the jar becomes sealed meaning that nothing can go into the jar and nothing can come out of the jar. This is the same thing that happens when a person chooses to leave their life of sin for a new life with Jesus. They are no longer an open jar susceptible to having unclean spirits enter them, but rather a jar that has been sealed by the Holy Spirit. This also means that once you have received the Holy Spirit, it will not leave you. You are a jar whose contents is the Holy Spirit and whose lid is the Holy Spirit.
"My sheep hear my voice, and I know them, and they follow me. I give them eternal life, and they will never perish, and no one will snatch them out of my hand." Jesus said this in John 10:27-28. Once we are a part of Jesus' flock, we will be with him forever. He will not lose any of his sheep. Part of being in Jesus' flock means having the Holy Spirit in you. So as long as you are in his flock, you will have the Holy Spirit and since Jesus said that none of his sheep will be taken from him, we can be confident that the Holy Spirit will never be taken from us.
We read in Hebrews 6:4-6 "For it is impossible, in the case of those who have once been enlightened, who have tasted the heavenly gift, and have shared in the Holy Spirit, and have tasted the goodness of the word of God and the powers of the age to come, and then have fallen away, to restore them again to repentance, since they are crucifying once again the Son of God to their own harm and holding him up to contempt." I used to think that this was referring to people who lost their salvation and therefore the Holy Spirit, but since Jesus said that none of his sheep will be taken from him, I have had to reconsider. Now I believe that these verses are talking about people who have heard the gospel and who have seen the power of the Holy Spirit in others, but still choose sin that leads to death rather than repentance that leads to salvation.
As Christians we need to be very aware of the demonic realm, but we need not be concerned about becoming possessed. The Bible makes this very clear. In a sense you could say that we have been "Signed, Sealed, and Delivered" -- Signed into the Book of Life, Sealed by the Holy Spirit, and Delivered from sin!