Is it possible for a Believer to be demon possessed? That's a question sometimes asked in a discussion on "deliverance". The answer is "No!". It may seem to be by the troubled person, but it's a demonic lie.
The absolute truth for everyone who believes, who has repented of sin and received Christ is this: our heavenly Father "has ...
... delivered us from the power of darkness and conveyed us into the kingdom of the Son of His love,
in whom we have redemption through His blood, the forgiveness of sins," Colossians 1'13-14.
He has delivered us. And that's the truth. Be assured. But, having said that, a Believer who is really troubled may be deceived by the enemy into thinking otherwise.
Because of this powerful understanding complete deliverance from the deception is always available. Believers have been placed in "the Kingdom of the Son of His love". Amen!