One way to isolate a single central theme as a means to discuss the several topics of the novel is to point to the notion of liberty beset by ideology in 1984.
While the people of Oceania are not in prison and are free to make certain choices about how to live their lives, they are functionally imprisoned - especially mentally.
At his most radical, Winston writes in his diary:
Freedom is the freedom to say that two plus two make four. If that is granted, all else follows.
When such a simple statement of fact is the extreme limit of this man's mental freedom, we have to question the nature and quality of Winston's actual liberty under his government. Orwell's novel explores the conflict between the individual's intellectual freedom and the interests of a totalitarian political state - interests which necessarily oppose any and all mental freedoms.
Winston is not free to think and act however he likes. He understands that the behavioral and intellectual expectations of his society (dictated by Big Brother) are more than merely conventional norms - they are the law and they are strictly enforced.
Winston, like the other citizens of Oceania, is a subject of a totalitarian regime, which is to say that he is subjected to its ideology. The aims and posture of the Big Brother regime serve to protect the state while also defining it.
The misinformation, reflexive moral/political justifications and the many practices of brainwashing and enforced ignorance all also function as the character of the government and as means to defend the integrity of that government.
This is ideology at work in its most complete and obvious form. Set against a framework of emotional, social and intellectual totalitarianism, the central (political) message of the novel seems to be the suggestion that individual liberty is impossible when the very foundations of thought are corrupted by political machinations (propaganda, cult behavior, fear mongering, etc.).
"The concept of free will, an individual’s liberty to make his or her own choices, is a threat to the totalitarian state. This is presented throughout 1984 as free will in one’s actions, thoughts, and information. Free will encompasses not only a choice, but also a choice made on the access to true information" (eNotes).
Seen in this light, 1984 presents a central theme of liberty imperiled by ideology. The ideological symptoms of a totalitarian state are precisely equal in their totality as the state is in its oppression of freedom. These symptoms are, in essence, the full and natural expression of the political body.