And why would that be? Inspect his clothing for any signs of him working for the cultists, or even being one of them.
He looks... Fancy. There's really no other word for it. He doesn't look especially rich, but he's wearing a nice suit.
Help him, and make some nice small talk, perhaps ask more about these cultists religion, and if they have any connection with strange voices, you know if you want, I'm quite curious where I come from actually, sure it could be your mind, but a case of multiple personality disorder with billions of personality's? Seems unlikely.
You grab one side of the chest. It's pretty heavy. You guys manage to lug it up the stairs, and after a quick look you go out onto the street, towards his car. "So, I've been tracking these guys for a while. They're pretty dangerous I think, murder and such. I don't know, I really only know about cults and stuff, nothing about fighting. They stole this from me by the way, I collect this stuff. " Together you drop the chest into his cars trunk. He shuts it with a thud, turns to you, and hands you a business card. "Drop by my house tonight, we need to talk. You're involved now, wether you like it or not. These are dangerourous people, come and see me."