You can find the answers right here at eNotes. I've pasted links to the Historical Context and Themes sections below. The Critical Overview (also linked below) notes that Equus is "a new and interesting example of the way sex can get mixed up with religion, or vice versa" and that some critics
have evoked the cultural associations with the Greek gods Apollo and Dionysus as a way of contextualizing the play’s intellectual conflict between subdued rationalism (widely viewed as normality) and passionate instinct (viewed as insanity). Lambert wrote in Drama that in this, Equus focuses on "a theme constant throughout human history, never resolved, always relevant, and very much in the air today."