My :twocents: …
If you think of the soul as a race horse, then the indwelling of the Holy Spirit in the soul is a race horse with a jockey in the saddle. The Holy Spirit gently guides the soul which He indwells. However, just as a race horse may choose to ignore the jockey’s commands or choose to throw the jockey off altogether, a Christian can choose to ignore the gentle guidance of the Holy Spirit through venial sin or choose to kick out the Holy Spirit from the soul altogether through committing mortal sin.
Or again, if you think of the soul as a homeowner, then the indwelling of the Holy Spirit is a homeowner who has invited a guest (the Holy Spirit) inside his home. Just as a homeowner can listen to his guest or kick his guest out of his house, the soul can ignore the Holy Spirit through venial sin or kick the Holy Spirit out of his soul through mortal sin.
If you think of the soul as a homeowner, then demonic possession is a homeowner, who has been overpowered and tied up in his own house by a robber, who uses the house for his own purposes. Just as the helpless homeowner cannot regain control of his house until the robber leaves of his own accord or is himself overpower by someone stronger, the soul won’t be dispossed of the demon until the demon leaves of his own accord or is overpower by someone stronger, like an exorcist working in the name of Jesus.