.45, you're exactly right when you say that a person indwelt by the Holy Spirit cannot be demon possessed. Satan and his sadistic servants cannot enter a person's soul unless that person in some way opens the door: gives them the key or the password, so to speak. Satan cannot prevail "from the outside" without help "from the inside."
However, all religious groups have persons who are not in a state of Sanctifying Grace. The mere emptiness of an unsaved soul, the vacuum itself, attracts demons.