Have you ever said the most absurd thing to your friend full knowing they'll obviously realize it is just a joke and they literally take it as truth and need you to convince them with great effort that it was really just a joke?

Then, you say something really small to them and they think you are kidding.

This is exactly how it works.

Quite ironic really, because Goebbels never actually said that. No one has ever been able to cite a source for it.

He did however say this:

“Good propaganda does not need to lie; indeed it may not lie. It has no reason to fear the truth. It is a mistake to believe that people cannot take the truth. They can. It is only a matter of presenting the truth to people in a way that they will be able to understand. A propaganda that lies proves that it has a bad cause. It cannot be successful in the long run.”