That’s the big twist of Apostle. The only reason Erisden — a remote, desolate island — is inhabitable is because the goddess has been making the land fertile. When she places her fingers on Thomas’ temples, the goddess transmits a flashback of the men’s early days on the island. Quinn was the one who discovered that if the goddess is fed blood, she would make the land fertile. So, the men put her in captivity and feed her animal blood for years. When animal blood stops working, Malcolm begins feeding her his own blood (that’s what he’s so casual about slicing open his palm at the beginning of the movie). Eventually, Quinn decides the only sustainable solution is by feeding her humans.