The second act, set in the hospital (replete with painful-looking spinal tap) suggests the latter. And then the third act goes nuts with violence and graphic effects, but also examples of human heroism and nobility. The film thus provides two answers about life and the cosmos. The first is that if there is a Devil there must, by implication, be a God too.
And secondly, that acts of human courage and kindness can, finally, beat the Devil.
The Exorcist may be brutal, but it is not vile. On the contrary, it suggests that where evil exists, good exists too. And that, finally, mankind can make a difference about which force succeeds.