Pretty sure Preacher is firmly within the Southern Gothic tradition, and as such is about people being flawed, disturbing or eccentric in a context of sinister or grotesque events relating to or stemming from poverty, alienation, crime, or violence. (Cribbed from Wikipedia.) Where, say, True Detective despairs of people and leads with nihilism, in Preacher there's a strong undercurrent of amusement or enjoyment in people being weird. We the viewer have the God's eye view. And as the show goes on, I expect we're going to shuffle between views. That's to say, I'm moderately sure the show is about temptation.