Very true. Actually, most if not all people have demons connected to their particular sins. However, this level of demonization is mostly temptation, and sometimes illness, rather than resulting in the kind of mental derangement that people think of as full "possession."
To avoid that, you have to do two things when dealing with a hefty demon. First, make sure that none of the team are currently walking in sin, and that everyone has confessed and repented of their sins. This keeps the doors that demons use to get in closed. And second, from the outset you bind the demons from attacking the ministry team.
According to these exorcists, the biggest risk factor for demonic possession is the victim's previous involvement with the occult.
That's where the more powerful possessing spirits tend to come from, yes. You can get some pretty hefty ones from sexual sins too.
Additionally, exorcisms can become violent. It can take several grown men to hold down a possessed person.
That's why, unless your hand is forced, you conduct a deliverance session in a group. Binding the demon from the first will almost always prevent this, and if they do manifest with supernatural strength, you can bind it on the spot then.
Having additional people is also useful for preventing the demon from causing self-harm to the afflicted.
My mentor's experience (20 years worth) is that in people who come forward and say that they want deliverance, the demons tend to go pretty quickly and neatly without a lot of fighting back. It's in those who won't (or are so far gone that they can't) directly ask for deliverance that you get the most manifestations.
I've seen footage of non-Catholic exorcisms where the demons have jumped into the people praying over the possessed person.
I've heard of this happening in Catholic exorcisms too. Basically, this stems from a couple of things. First, failure to bind the demon against going after the deliverence team at the start. Second, someone in the team having an open door or selfish motivations. Or third, I've heard of cases where the person doing the deliverance got discouraged with a particularly strong or tenacious demon and offered to let it enter them if it would leave the first victim alone. (I seem to remember the movie The Exorcist ending that way.) That simply doesn't work, and since they entered with full permission, getting them back out again can be very difficult (though not impossible).
Of course, you never do deliverance on someone without their permission, or the permission of their parents/guardians (in the case of children and the incapacitated), unless God were to specifically tell you to (which is very, very rare, if it ever happens). You also never do it on someone who is not a Christian. Doing so just opens the door for seven more.
An interesting story about that: Back while he was still learning about all this (back in the days when nobody spoke or wrote about it), my mentor received a phone call from someone who said she was possessed and threatened to kill herself. Without first asking if she was a Christian or was ready to become one, he did the deliverance over the phone. When he was done, he said, "Okay, now let's talk about Jesus."
"No, thanks," she said. "I'm a witch, and I wanted you to get rid of that demon so that I could get seven more and become more powerful."
With territorial spirits (the kind that haunt houses instead of people), you likewise don't go after them without the property owner's permission unless God tells you to go ahead (as He did in a particular instance just the other day with me and my partner). If it's yours, you don't need anyone else's permission, of course. In most cases, there's a particular curse that needs to be broken before the demons can be expelled and the property annointed. Sometimes you can find out the curse by checking that property's history. Sometimes God will tell you directly (as He did with us the other night).
Breaking curses isn't any more difficult than casting out demons, by the way. You don't have to have any special key other than the knowledge of the curse--after that, it's just breaking it in the Name of Jesus (who after all became a curse for us when He hung on a tree). Then you kick out the demons associated with the curse and command them not to return.
In any case, the territorial demons tend to be pretty big and nasty, and it can take a while to get rid of them even after the curse is broken. On the other hand, you don't have to deal with a person thrashing around and screaming, so in some ways it's easier.
You have to be careful and in the Lord, but you don't have to fear fallen angels, demons, and occultists. On the contrary, if the Lord is for you, who can be against you?
Whoa.
Great post, and God bless your work. I don't think I have the stomach for it myself.
FYI, I think the Church limits exorcism to the clergy because of the grace conferred by Holy Orders, and because, like you say, the exorcist has to be trained to resist the devil's tricks, the major one being tempting the priest to rely on his own strength rather than Jesus.
If you haven't read it, I highly recommend Malachi Martin's "Hostage to the Devil." Scary as hell though.
"An Exorcist Tells His Story" is also worthwhile.