People of Christian faith can still be afflicted or persecuted/oppressed by demonic people, energy or spiritual forces.
Just like a police officer may be authorized to enforce the law, but can still be attacked unlawfully and even injured or killed.
Priests who get too far entangled in the deliverance/exorcism process HAVE been attacked and even physically injured by demonic entities.
Possession is generally rare, and most cases of demonic influence are OPPRESSION.
Since full possession is rare, it is even harder for a Christian believer to get that far in trouble with dark forces.
Any true Christian believer would avoid entanglement with dark/occult forces, and would go get prayer help
with deliverance and exorcism before it got to the point of full blown "possession."
The believers I know who understand enough of the demonic forces to seek or invoke exorcism/deliverance prayers,
HAVE experienced threats and attacks by such spirits.
Uniting in prayer for protection in Christ Jesus Authority to cast out these forces and entities
increases the safety net. But where we don't fully forgive and resolve conflicts, or let
our personal agenda or ego get in the way, the negative forces can use those weak spots to
get in and start trouble.
We can get more help when that happens,
but just being a Christian believer isn't enough to prevent attacks.
the attacks are an expected stage in the process of deliverance and healing.
It is commonly known that these demonic forces will seek to attack the
Christians helping the host person that the demons want to keep using like a parasite.
So the natural reaction to the introduction of the healing prayer is for the
demons to go attack the people on the healing team and try to invade any weaknesses in faith or ego.