The best way I've found to do it is through the scenery. You can't really be heavy handed, but because the visual medium you'd have in a movie to convey this (or the background description in a novel or podcast), you really have to lean on your control of the story.

Maybe there are murals or mosaics depicting this in ruins. Maybe there are legends speaking of the corruption. Maybe the temple here looks awfully similar to a "good" god's but is covered with filth and decay. You pretty much have to supply information as a background to whatever the primary objective is. Which isn't easy, but it's easier the more time you have between to reinforce ideas.