If Adam doesn't know how it happens, then it's pointless. How can they fight something like this when they don't even know what form it will take? He believes Adam when he says that he'll stop it, but Ronan doesn't know how, and that in itself is terrifying.

But Adam promises him that he'll fix it somehow, and it's hard to doubt him- even though he knows that Adam lies sometimes, that Adam's jealousy can lead him to rashness and anger- he can't believe that Adam wouldn't stop at nothing to save Gansey.

He still wavers, but in the end, when Ronan makes a decision, he makes it. Adam has to get to Glendower, has to ask for the favor, has to save Gansey. He will- it's the best shot they have right now, he thinks, and if anybody can push through on sheer grit and determination alone, it's Adam. So Ronan nods, his jaw tight, resolute.

"I believe you."