Descartes didn't tell us to believe anything. He just questioned everything including his own existence. From there he said the only thing he could be sure of was his consciousness since he could question his own existence. Everything else is a mystery.
This isn't me believing in the word of a cac. Its me performing the same thought experiment and coming to the same conclusion.
All we can be sure of in this world is that we are conscious. Everything else could be fake.