I did this one in '82 or '84.
This looks like a copy from the newspaper ad, the original was red and black.
I had forgotten about this poster until last week when I was watching "The Pick-up Artist" on VH1. Filmed in Austin, there was a scene in a hipster clotheing store that had a framed copy of the poster on the wall.
Its got to be a church of some sort - pretty much all churches follow a vague cross layout with the altar being in in the top part. Like this one http://www.naturalhistorymag.com/images/crossection_full.gif
This one has that little atrium addition which makes it look like a key. Not sure if its intentional or relevant, but it could be trying to play on something like that. Its a good looking poster that fits the vibe.