Maybe the cultists have another faction that they’re rivals with and the leader is impressed by the party’s ability to sneak in? So he makes them an offer - do a similar job against the rival faction and if successful, he’ll release the captive to them.
They can then either do the new job or try and bring the mob down on the cult. Or some kind of double cross later? Plenty of opportunities for shenanigans or they can play it straight and try and to do the new job.