Ancient Kingdom Time Forgot

It's not really that simple, all of this is built on the very first basic premise that Klein was able to whisk everyone away from wherever they were and gather them above the fog. This was the most unshakeable foundation upon which Klein built a lot of b*llsh*t which makes it possible for others in the LotM world to believe him more easily. His first move itself was out of their scope of imagination so the follow up is understandable.

In the whole LotM world that was shown in book 1, THO was the only other organisation which was able to have a gathering as supernatural as Tarot Club so it makes sense that the other members were easily accepting of Klein's words.

Saying that the TC members easily accepted anything Klein said just like that is doing disservice to characters like Hanged Man, Magician, Hermit and Star. While some members are truly gullible towards Klein (Justice, Sun), there were also others who were actually careful about such matters.