That "reveal" stinks of red herring to me. I think they want you to think that, but she's actually doing some sort of psychological trick to try to "bond" or get close with the guy. She is the head of the FBI's "Cult" division, she probably has a few tricks up her sleeve. I'm pretty sure there is an agent cultist, I just don't think it's her. To me, it's more likely it's the younger guy, who was waiting outside the cult house for Bacon. He probably had a key, just let himself in, then attacked Bacon, then took the mask back off and "vanished."