How to unlock: Speak with Casmeen and Mimeon (M3.2) after you finish Fallen to Earth main quest.
This quest is not easy to unlock because it has a few additional requirements. The most important of them is to say that you are not The Changing God during the first meeting with Casmeen and Mimeon in Circus Minor (M3.2) - it is the second option that can be seen on the picture above.
When you inform the cultists about your identity, finish Fallen to Earth main quest which means that you have to help with the repairs of clock. After that, meet with Mimeon and Casmeer again and, during the conversation, ask them if you can join the Cult of Changing God (the picture above).
In this quest you must identify one of the Castoffs who is yet unknown to the cultists. This can be done by finishing other side quests that are available in Sagus Cliffs. The first quest is The Sorrow's Prey, unlocked when you speak with Zaofi the Sculptor (M3.9). During this quest you meet with Seria. From her you can learn about her father, Orseolo, who was one of the Castoffs.
The next Castoff, who can be met during Beloved Slave quest given by Tol Maguur (M3.14), is harder to identify. During this quest you must bring Rhin to Tol Maguur. After that, you must lead the conversation with Tol Moguur in such a way that she reveals that she is also a Castoff. More information in the part of the guide dedicated to this quest.
Regardless of which of the Castoffs you met, return to Mimeon and Casmeen and tell them what you have learned. You receive 40 XP and from now on you can rest in the Cult of Changing God's camp for free.