I thought maybe I would go back for all the treasure I left behind...

Well, I don't see why you couldn't go back by the main entrance, through the chute...

Yves

What is stopping you from going back in the way you came out. If you exited via the passage behind where the priestess was standing, you will have walked back to the room just above the library. Lets see, going from the snake idol / brazier room, take the right fork up. You come to a room - where turning right leads down to the library, and it's straight ahead for the priestess room. Before the fight with Elyssia you couldn't get through this way. The lever behind her opened they way. I wandered back in here once after leaving the temple, so I know it stays open for you.