You have to see it to believe it.

Said of that which is or seems so extraordinary, unlikely, or surreal that a person would only believe it if they witness it for themselves. I've never seen a magic trick like it! First she encloses the volunteer in the box, then she wraps the box in chains, hangs it upside down, sets it on fire, and chainsaws it in half—only to reveal that it is totally empty! I'm telling you, you have to see it to believe it. The company is promising a resolution on their new television set that is so sharp and detailed that you have to see it to believe it. The film's finale is incredible, a mind-bending audiovisual spectacle. You have to see it to believe it.