I think overall, The Exorcist is a film in denial of itself, with everyone (including the priests) trying to find an explanation other than possession. In that respect, it is symbolic of post 1960s American society, urging it to reconnect with spirituality. But yes, whilst I do believe sometimes only God has the answer, medical science (and science in general) exists for good reasons and shouldn't be demonised (pun intended), even by a film as excellent as The Exorcist.