I've found this season (focusing on the island and Andy's foster home) to be a well thought-through and in several ways uplifting allegory on the "demon" that is depression (this episode's title, in retrospect, is a hint): from the way the disease takes its insidious hold on the sufferer, to how nebulous, disorienting, isolating and corrosive it is, to the way various characters take solace in and strength from one another.
The other part of the season, focusing on the characters Bennett and Mouse, almost seems like it's only there to appease those expecting something more obviously related to Blatty's original "Exorcist". In that sense, I'm not sure this parallel plot succeeds, as it feels more Dan Brown than William Blatty.
Still, very worthwhile viewing, with some great performances and imaginative cinematography throughout.