The narrator is also an actor. He's up to his neck in the adulteries and the suicides. Who's he trying to fool with his protestations of hapless ignorance?
Was anyone ever so innocent or innocuous? So much of a booby? And did you notice how this drawling, Jamesian slacker beat up on the avuncular, black manservant when he thought our attention was otherwise engaged?
So it's about truth. Trustworthiness. Objectivity and subjectivity. It's also about decadence. And sex. And Englishness. And men and women. And ethics. And catholicism.
There are those who say it's the greatest novel in the English language. I need to mull it over a bit more but, yup, I could well be joining them.