... only Roman Catholic priests have the real power to expel the demons.
Have any of you heard of this before?
The question of whether or not only Catholic priests can exorcise demons is small potatoes compared to the bigger picture.
If you believe this, you need real demons. Then you need a demon or two capable of possessing a human being. Then you need a real God. Then you need a church chosen by God. Then you need priests with God-given power to exorcise the demons.
I smell a lot of "if" coming off this idea. Mental illness is the simplest and most reasonable explanation for "demonic possession."