That's take directly from Wikipedia. Funnily enough, Wikipedia accidentally lets in some based facts.

The following supposed quotation of Joseph Goebbels has been repeated in numerous books and articles and on thousands of web pages, yet none of them has cited a primary source. According to the research and reasoning of Randall Bytwerk, it is an unlikely thing for Goebbels to have said.[16]

If you tell a lie big enough and keep repeating it, people will eventually come to believe it. The lie can be maintained only for such time as the State can shield the people from the political, economic and/or military consequences of the lie. It thus becomes vitally important for the State to use all of its powers to repress dissent, for the truth is the mortal enemy of the lie, and thus by extension, the truth is the greatest enemy of the State.