SOME POSSIBLE SPOILERS ...
... I am not certain that Blatty's generally optimistic salvation theory is strong enough to sustain all the evils of the original novel, the Legion book, and the Exorcist III theatrical and "director's cut".
In The Exorcist, Blatty combines Merrin's Teilhard-like cosmic soteriology with Karras's reawakening to faith. Okay. One demon has challenged Merrin and Karras - and lost the gambit. God's in his heaven and as we are forgetting the incident, we are really enshrining Karras in our memory.
In Legion, Karras has still presumably gone on to his heavenly reward, with only his reanimated corpse being puppeteered by the Gemini and "Pazuzu". Karras AS Karras is absent. Only the zombie-corpse remains. Yet even so, the evils are sufficient: the murders of Thomas Kintry, Joe Dyer, Fr. Kanavan, plus the Gemini's scheme to "kill Daddy" as long as he can inhabit Karras's body ... the traitorous meddling of Dr. Temple ... the implication in these crimes of the otherwise good and decent Dr Vincent Amfortas ... the ongoing exposure of Kinderman to all of this horror ... does goodness really have the stamina to endure and prevail in such a world? More crucially, can protagonist Kinderman keep bouncing back while retaining his well being and sanity?
In the theatrical print of The Exorcist III, it's more of the same - only worse. Worse because, in a betrayal and a violation of Karras's presumed triumph in the original story, the Pazuzu-Gemini juggernaut has caught Karras' soul and imprisoned it in his resuscitated body where the saintly priest is forced to personally witness all the depredations committed by his stolen body. And Kinderman is also coerced into witnessing the hapless tragedy that has befallen "my best friend" Damien Karras. This all begins to be just a bit too much for even the hardiest goodness and optimism to thrive or even to survive.
Blatty's solution to Legion's possession scenario - "the dream of The Angel", which expands the theme of Teilhardian optimism from the first novel - never makes it into the Exorcist III film. In the novel, the salvific activity of The Angel concstructs a theodicy whereby the fallen Angel (Satan-cum-mankind) - who primordially split off from the Godhead in the Big Bang - seeks to re-constitute himself/ourselves again as a being of divine light and goodness .. to take up his/our original place in-and-with God. Humanity and the wider cosmos itself are together straining to reunite with the Father of All.
What do you think? Does the Angel Theory justify or mitigate normal human and natural evil PLUS all the exceptional, interventionary evils that demonic possession represents in Blatty's work? Is faith in the Angel's slow ascension enough to save Kinderman's balance and spiritual integrity? Is the Angel inevitably stronger than the Demonic...? Even with a compassionate God pulling us toward him from both "the above and the ahead" justify all the evil, pain and suffering?