I'm by no means an exocrist ok, and I think I might've already given inaccurate information on this topic, but after further research it seems "you don't have to open yourself up to the demonic to become possessed". Demons can take possession of 'parts' or the entire body, it can be isolated. It's false that you have to give consent for the possession to happen and it's also false that you have to do something evil to become posessed.May I suggest you read 1John 4:4, Ephesians 2:2, Colossians 1:13-14, 2 Corinthians 6:15, Ephesians 1:13-14, 1 Corinthians 6:19 and finally Romans 8:38-39 (all KJV). There is no example in scripture of a believer in Jesus Christ being demon possessed, all cases of demonic possession were with those who were unbelievers. Be careful not to rely on stories of believers and demon possession, many other explanations are possible. The Christian who is indwelt by the Holy Spirit, should not fear demon possession. Having said that, demons do attack believers in various ways. Once again (in my humble opinion), oppressed yes.... possessed no. In peace, God bless.
You can become possessed apparently by:
1) committing a mortal sin
2) something gravely disordered happens to you
3) the will of God
People that are partially possessed can generally function normal and might not even know it.
Full possession the demon is in control all of the time. But with that being said apparently it doesn't look the way you think it does.
Anyways, it's a complicated topic. If you think simply 'the Holy Spirit is in me I'm fine' then I think you're missing the overall point of what's actually happening. The demonic is on a short leash. God allows demons to oppress you for your santificaiton, for you to grow in virtue.
Maybe you don't care for the Roman Catholic answer on this, flip to the Book of Job. You might say, "He wasn't possessed." But was he not under incredible oppression which most people will never experience? Of course he was.
Not to mention, but apparently quite a few possessed people are some the holiest people. For them to even have a normal functioning life, they have to strive after holiness far more than the average person. Anyways make of it what you will, there's quite a bit more on the topic, even from a scriptural point of view which I think you're overlooking, but goodluck regardless.