The party told you to reject the evidence of your eyes and ears. It was their final, most essential command. His heart sank as he thought of the enormous power arrayed against him, the ease with which any Party intellectual would overthrow him in debate, the subtle arguments which he would not be able to understand, much less answer. And yet he was in the right! They were wrong and he was right.
The mistake Orwell made was in thinking that the Ministry of Truth even needed to go back and edit old press releases and announcements when we've now seen they could have just shouted FAKE NEWS and a good chunk of the country would have went along with it.