Here is the thing. To say a believer can be demon possessed is to say that a demon can co-habitate with the Holy Spirit!
The Holy Spirit IS God. The Holy Spirit is faaaaaaaar more powerful than a demon.. even Satan.
So.. the Holy INDWELLS a person He has saved. The Holy Spirit will actually be physically in a person's soul, sealing them unto the day of redemption so they go to heaven when they die.
Of course a demon can AFFECT a believer.. because every believer still has a fallen nature. They can still sin, be tempted etc.. but that is not possession.
The Holy Spirit will protect and shield a believer.. and there is no way any demon or Satan can inwell a believer with the Holy Spirit there.
It's like saying an ant can co-habitat with a mega size ant eater!
The demon being the ANT.