To Save The Sinless is a Lightning Returns: Final Fantasy XIII missable quest with specific requirements that must be met, in order to play it.
To Save The Sinless is triggered as a result for reading Ranulph’s Journal while playing The Avid Reader and it can’t be completed if the journal is returned without reading it.
In other words, To Save The Sinless becomes available as a consequence for breaking Ranulph’s privacy.
Additionally, To Save The Sinless requires a series of additional side-quests you can play in Luxerion, and the following guide reveals all steps you need to follow if you intend to play To Save The Sinless.
Requirements: Start at 5 AM, and complete the quest before 11 PM. Fail The Avid Reader, complete Buried Passion and Born From Chaos
Rewards: 4500 Gil, Maximum HP + 120, Strength + 12, Magic + 10, Dark Knight Garb
As outlined by the The Avid Reader walkthrough, once you get Ranulph’s Journal at the end of the quest, you will be asked if you want to read it.
If you answer no, and return it, The Avid Reader is completed, while To Save The Sinless remains locked.
On the other hand, if you answer yes, and read the journal, once you return it to its owner; you will fail The Avid Reader, but unlock To Save The Sinless. Assuming that you read the journal, go back to Ranulph and watch the cutscene that unfolds.
If you finished both quests, return to Armand, the client from Buried Passion and talk to him twice.
Next, go back to Reddick in the Wall of Joy sub-area of the Warren, after midnight.
Once you talk to him, all you have to do it is to return to Ranulph, to get the Dark Knight Garb and all associated rewards.