And what about Cassiel couldn't he tell them about what is really going on?
Cassiel doesn't want humans and Gods to be involved at all. If they have found out about The One, then that's their decision to worship/revere him. Cassiel himself doesn't want Gods to have any part in humanity, and if he teaches others about The One, then they would worship him. Cassiel also believes in human empowerment, and will not intervene with his nation in any way that a normal human could.