Roleplaying blues

I'm at a loss as to what to do at this Thursday night's gaming session. My character's on a quest to obtain an ancient artifact and bring it back to his people (it was originally theirs, or rather the property of one of their gods, but was stolen and taken to another country). I was basically expecting a search-for-the-long-lost-doohickey situation, but when the group got to the capital city of the country where it was taken, it turns out it wasn't long lost after all. In fact, it's on display in the front of the temple of their god*. One of my allies is a priest of that god. And he's starting to question why he should be helping me do this, since it'd essentially be stealing his people's prized artifact (see, to them it's the weapon of their legendary hero, and they don't think of it as stolen, more a "spoils of war" sort of thing).

There have been hints that the one on display is actually a fake (I won't go into details, because I'd have to describe a lot of the game world and it'd take forever). That means grabbing & dashing isn't really an option, which is just as well since my guy is a (rather weak & lousy) warrior, not a rogue. But that doesn't help with the main problem, which is balancing accomplishing my quest with not pissing off my teammate.

I'm completely at a loss.