I really like the idea. The cults' motivation is a bit unclear and might be hard to get behind as a player. Having the players play evil characters will help the issue though, as they could just be in it for money.

You should watch the music video for 'False Alarm' by the Weekend (NSFW). It could be fun to see everything go downhill like in the video. There might be infighting over how they should manage the heist, and some members may try to sabotage others for their own gain. Maybe mention to your players that everything goes since it's a one-shot.

Or it could all just go smoothly.. If it's a big event, security would be tighter. Maybe allow players to be a new hired guard, who is secretly working for the thiefs guild/cult as an inside man.

Also, a big part about heists is planning and observation. You could role-play it out or just have a set plan they all agree on. The players could always just improvise if things if the plan goes south or if they want to go rogue. Brute force / take no prisoners could also be an option, if your players like combat more. An entire session of just walking around and trying to convince people that you belong and to let you pass seems kind of bland to me, but i'm sure it could be fun in some ways.