What if the PCs get to the damsel and realise they've got it all backwards, and they are the ones being rescued?
Maybe the cultists have purposely drawn the party into the temple, hurling themselves at the PCs and having them face all sorts of dangers as a test (perhaps they're seeking a worthy vessel for an avatar of their god, and have been drawing adventurers in as rather deadly job interview), using the damsel as a carrot to attract their attention.
Unbeknownst to the cultists, however, the damsel is actually an envoy from a god trying to stop this from happening, making herself appear in distress in order to sit at the very heart of the cultist operation, ready to act when a worthy group of adventurers finally manages to get there and thwart the Reptile God's plan. She can't warn them until the ritual starts, because she'd be tipping the cultists and maybe the whole process needs to start in order to weaken the Reptile God before striking.
So she has to play her part until the very last moment, when she can reveal her true intentions and assist the party by way of warning ("Once you kill the High Priest, his soul will attempt to enter your body! Drink this when it happens so we can trap him forever!").
Alternatively, the damsel/envoy could be waiting to slay the PCs instead after the ritual has been completed, as a way to stop the Reptile God while he's trying to manifest in one of them.