Final answer:
Without a direct passage from Rick Riordan that explicitly mentions Cronus, it's challenging to determine his exact expression about the Titan. More context from Riordan's work would be necessary for an accurate interpretation.
Step-by-step explanation:
The key idea that Rick Riordan is expressing about Cronus in the provided passage appears to be primarily related to the mythic themes rather than the specific character of Cronus himself. Riordan seems to be interweaving classical mythology with his own narrative, creating a reinterpretation of the myth for modern readers. However, without a direct passage from Rick Riordan's work that references Cronus, it is difficult to provide an accurate interpretation of Riordan's expression about this particular Titan. The excerpt from Celsus about Jupiter may not directly relate to Riordan's depiction of Cronus, as Celsus's critique is focused on Christian theology rather than Greek mythology. More context from Riordan's writings would be needed to determine the key idea he is expressing about Cronus.