The big lie is the propaganda method where an outrageous lie —like flat Earth— is said and repeated as if it is true, hoping people will assume it is true for granted rather than critically questioning it.
Even though anyone can easily verify that Earth is a sphere from simple personal observations, many still fall for the notion of a flat Earth. The reason is that it is such a big lie and does not fit their model of modest deception, so they think “it must be true.”