Take one of the slogans of The Party and analyze its meaning (WAR IS PEACE; FREEDOM IS SLAVERY; IGNORANCE IS STRENGTH). What is The Party trying to get the people to believe?

"Who controls the past," ran the Party slogan, "controls the future; who controls the present controls the past." (Page 34) 

The Party is trying to get people to believe that the people who control now, know what the past is. This quote can be a tad bit confusing but Winston mentions that the Party controls the past. The Party makes up a past and Winston believes that they are lying. For example, on page 36, he says that the Party lies about inventing airplanes. The first part of the slogan is somewhat true. It's true because what you do now will later affect you. So, you control your future with what you did in your past.