Can Christians be demon possessed?

That really depends on the definition of what a Christian is.

From my understanding, a demon cannot possess someone who has the companionship of the Holy Ghost. Determining whether someone has the companionship of the Holy Ghost differs between denomination.

There is a distinction between being a believer and having the companionship of the Holy Ghost. Receiving the Holy Ghost is distinct from baptism, and occurs afterwards. Here is some evidence to support that:

• Acts 2: 38- the Holy Ghost comes after Baptism
• The Apostles received the companionship of the Holy Ghost after Christ's ascension
  • John 7: 39, John 16:7- The Holy Ghost was not present during Christ's ministry
    • Note that the Gift of the Holy Ghost != the power of the Holy Ghost, otherwise nobody could have received a testimony (1 Cor. 12:3, Matt 16:16-17)
  • Acts 2: 1-4- the Holy Ghost poured out on the Christians on the Day of Pentacost after Christ's ascension
• Acts 8:12-17- The Holy Ghost is conferred through the laying on of hands
  • Acts 8: 18-20- To give the gift of the Holy Ghost, one must have power given to them (the method by which this is done is debated)

If having the gift of the Holy Ghost is necessary in being "Christian", then no, Christians cannot be possessed. But if just having a belief in Christ is necessary in being Christian, then Christians without the gift of the Holy Ghost are still susceptible, provided they are too weak to defend themselves against the adversary.

No one of the other answers take into account one thing: there is a big difference between devil's rule over soul (i.e. sin) and over body (possession).

Amorth refers cases of very devout people (some of them already died and were pronounced saint) who vere possessed for some time, but they didn't sin, so devil had control over their body, but not soul. Real possesion (see the list of forms of extraordinary demonic activity) is very rare about these saints, but many catholic saints experienced demonic oppression for some time or even for most of their lifes. Padre Pio is one of well-known examples of heavy oppressed saints. Blessed Mariam Baouardy is one of the few examples of really possessed saints - she was possessed for 40 days. I haven't found exact details online, but I have read in another book by Amorth (in Czech it was called "Vítězit Boží mocí", "To win by the power of God", but I haven't found how its English edition is really named) that the demon tried to leave her but wasn't allowed to - he controlled her body, but prayers of her spirit were hurting the demon even more than the evil spirit tormented Mariam.

2Cor 12:7-9a says:

7 To keep me from becoming conceited because of these surpassingly great revelations, there was given me a thorn in my flesh, a messenger of Satan, to torment me. 8 Three times I pleaded with the Lord to take it away from me. 9 But he said to me, “My grace is sufficient for you, for my power is made perfect in weakness.”

Whether or not the "messenger of Satan" mentioned in 2Corinthians is a demonic obsession or not is a matter of debate, but it clarify one of the aspects of "why some saints are obsessed/possessed". This happens mostly to mystics, who might easily go proud about special grace given to them through visions and extasies. The other reason is expressed in Colossians 1:24 - this is one of the sufferings a Christian my take to fill up "what is still lacking in regard to Christ’s afflictions, for the sake of his body, which is the church".

A final disclaimer: it might seem that being born-again doesn't protect you from extraordinary demonic activity at all. Full armor of God helps you in spiritual war against the demons. It's like being in a tank while most others are just infantry. In a tank, ordinary enemy soldiers are no match for you, but enemy anti-armor units who ignore ordinary soldiers will take you as a target. Ignoring the possibility of extraordinary spiritual attacks against new-born Christians is like assuming that enemy has no anti-armor weapons.

It helps to compare the situation with changes in mindset. We know that people who are selfish and immoral act out their worldviews, but some of these actions seem to exceed even the impetus the individuals assign to them. They seem to be reinforced by external forces that amplify these actions, turning them into monstrous deeds.

When individuals come to God, they have a sketchy idea of the relationship. Some look at it as a contract, a quid pro quo arrangement, a cosmic vending machine, a commodity provider requiring an input cost. The fact is, God allows a period of adjustment, a phase where doubts are clarified, after which a change in mindset is required, literally change of mind, metanoia, often wrongly translated repentance, in the sense of being ready to atone for sins.

The change in worldview is first, that God requires right behaviour and second, that right behaviour is possible only with God's help. Since both good and evil individuals are invited to the wedding feast, it is the latter who are required to change their mind, from might is right, to justice is right. It is the latter which makes men distinguishable from animals, reveals the image of God in men.

Jesus describes it like this:

Luke 11:24-26 NET “When an unclean spirit goes out of a person, it passes through waterless places looking for rest but not finding any. Then it says, ‘I will return to the home I left.’ When it returns, it finds the house swept clean and put in order. Then it goes and brings seven other spirits more evil than itself, and they go in and live there, so the last state of that person is worse than the first.”

Remember Judas? Ananias and Sapphira? They had the same opportunity as the other disciples, to learn, and to believe. Seems they refused to release their old world views, although they had kept them aside in following Christ. More important, their wavering, their attitude of sitting on the fence, left them open to a massive, reinforced onslaught of those old views, with disastrous consequences.

Luke 22:3 NET Then Satan entered Judas, the one called Iscariot, who was one of the twelve.

Note, satanic possession differs from satanic attacks. The first causes the ”individual” to do evil deeds, either harming themselves or others. The latter causes ”satan” to do the evil deed. Job and Paul both suffered satanic attacks.

You, dear children, are from God and have overcome them, because the one who is in you is greater than the one who is in the world. (1 John 4:4)

I have experience this verse myself when encountering a demon possessed woman, who had escaped from a hospital under psychiatric protection. When I encountered her, her eye’s locked into mine and I sensed an evil like breeze overwhelming me. Her horrid evils eyes grew excited as I fearfully thought that a demon (among many inside her) was trying to leave her and enter me. I was afraid and had no time to think.

Instantly a holy anger rose within me overpowering my fear and I can only describe what felt as a wave of peaceful anger flowing back against her. It was like a knee-jerk reaction. She turned her face violently away from me as though she had become frozen. I was a new Christian so I did not attempt to cast the demon out from her. I just left.

From this encounter, I have always read this verse differently.

Anyone who hates his brother is a murderer, and you know that no murderer has eternal life in him. (1 John 3:15)

Having also been demon possessed prior to my conversion, due to payers that I had made to the Devil during my teens, I can vouch that anyone demon possessed will experience hatred of others.

But all Christians have eternal life in them:

I write these things to you who believe in the name of the Son of God so that you may know that you have eternal life. (1 John 5:13)

So you see from scripture and first hand accounts, it is not possible by any means that a Christian can become demon possessed. Even a Christian who is so very much afraid of that, this fear can not lead to it actually happening.

Nowhere in the Bible is a Christian demon possessed.

I heard someone say that a Christian cannot be demon possessed because they are possessed (filled) with The Holy Spirit.

Since we don't appear to have any Biblical reference, I think that's that best answer we can give (unless anyone had a clear counter example).

Fr. Gabriel Amorth (at one time the chief exorcist for the Vatican) in his book, An Exorcist Tells His Tale cites two examples of people who were decidedly Christians (and, in fact were very devout) who were possessed by demons. He goes on to explain that it is his experience that possession by a demon has nothing to do with the state of one's soul.

He also makes the comment that the belief that somehow churches are immune from the work of demons is patently absurd. It is something which was made up by people in Hollywood. There is no place that is safe from demonic influence. There is no state where the devil is unable to torment us (though he also says that there are things we can do to make him less willing/interested in attacking).