The clue given in the message and in your quest journal is...
Secrets in the dark. An EXIT awaits you. To find it... Nocturnal's statue beckons you. A leap of faith tempts you. Submission is your friend.
With this clue you don't need to read any further... right?
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Nocturnal's statue beckons you - there are two hidden pull chains, one on each side of Nocturnal's statue. It's the same statue you initially worshipped
A leap of faith tempts you - go through the door into the Inner Sanctum. Jump into the pit. Just wait a minute, then something will happen
Submission is your friend - After the pit, get into the cage