if you believe that, you'll believe anything

if you believe that, you'll believe anything

That is obviously false. The phrase implies that if someone can't identify something that is so obviously untrue, they are apt to accept any falsehood. A: "My son said his teacher doesn't believe in grades, so they don't get report cards." B: "If you believe that, you'll believe anything!"
See also: anything, believe, if
Farlex Dictionary of Idioms.
See also:
  • if that's not (something), (then) I don't know what is
  • ninebob
  • phonier than a three-dollar bill
  • (as) bent as a nine-bob note
  • (as) queer as a nine-bob note
  • bob
  • dill
  • queerer than a three-dollar bill
  • (as) queer as a three-dollar bill
  • queer as a three-dollar bill