I'm searching for some clarification for the rule "Became a Believer" (Ch. 9 - Magic, page 179 of Italian edition CoC 7e manual).

The explanation in the manual is quite unclear. This is what I’ve understood:

Example PC: Harvey, Current SAN 60, Max SAN 99, Cthulhu Myth 0%

Myth Book: Cultes des Ghoul, Cthulhu Myth: +4/+8, SAN loss 1D10

Harvey does a first read of the book.

Keeper: “Do you believe?”

Harvey answers…

  • Case 1: “Yes.”

    Harvey loses 1D10 SAN: he rolls 4. He gains +4% Cthulhu Myth, max SAN 99-4 = 95.

  • Case 2: “No.”

    Harvey gain +4% Cthulhu Mythos, max SAN 99-4 = 95

    Harvey then reads some more books and gets a total Cthulhu Myth 10% and max SAN 99-10 = 89.

    Then some time later Harvey sees a Myth Creature, loses SAN for this and Became a Believer, Harvey also instantly loses SAN equal to Cthulhu Myth (10).

    Then all malus of huge sanity loss (>5) are applied.

So the pro of not became a Believer is that the PC doesn't lose SAN reading books but the con is that he will probably lose later, all at once, probably at a bad time.

Is that correct?


Your interpretation is correct.

This is all per the English language "Becoming a Believer" rules on pg 179 of the 7th edition Call of Cthulhu rulebook:

If the character chooses not to believe what was read, then the Cthulhu Mythos knowledge granted by the book is added to the investigator's skill and maximum Sanity points are reduced by the corresponding amount, but no Sanity points are lost. [...] However, when encountering evidence of the Mythos firsthand, the investigator will realize that those accursed books held the truth! At that point the investigator becomes a believer and immediately loses Sanity points equal to his or her Cthulhu Mythos score.

If Harvey-the-Unbeliever reads Cultes des Ghouls, he will gain +4% Cthulhu Mythos and have a maximum sanity of 95. If he then reads the Latin Necronomicon (2d10 SAN, +5/+11% Cthulhu Mythos), he'd have a Cthulhu Mythos of 9% and 90 Sanity out of 90. If he were to then encounter a Byakhee and lose 3 Sanity, he would immediately lose another 9 for a total of 12, leaving him at 78 Sanity out of 90.

Whether being a believer or an unbeliever is "optimal" is perhaps missing the point of Call of Cthulhu, but it should be pointed out that Sanity can be lost for non-Mythos related reasons, such as gruesome corpses, the violent death of a good friend, or being confronted with a phobia -- it would be very rare for anyone but a freshly created investigator to have full Sanity. In addition, an unbeliever is likely able to go longer without taking Sanity from seeing Mythos creatures as it would be easier for them to pass their Sanity checks.

  • \$\begingroup\$ Good thanks. On first read I misunderstood that the Cthulhu Myth sanity loss must be added to the the sanity loss of the book... And that looks quite evil, even for CoC :D. I agree that any discussion about "optimal" has no point in CoC, I asked as a keeper because i was feel something wrong with this rule at first. \$\endgroup\$
    – Tsumi
    Jan 18 '19 at 19:21
  • \$\begingroup\$ Something that the rules do not explicitly state (unless I missed it) but which may be relevant is that, presumably, if the non-believing investigator is already at (or very close to) their maximum sanity then reading a tome, even as a non-believer, may cause sanity loss, since mythos skill reduces maximum sanity whether one is a believer or not. So if Hank (SAN = 85, MAX-SAN = 87) reads a book that gives him an extra 6% mythos skill, he's still going to lose 4 SAN even if declaring himself a non-believer, because his MAX-SAN is now 81. \$\endgroup\$
    – RickL
    Apr 21 '19 at 13:35

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