He has some dreams that through his diary he can create a reality that will trump the one he lives in. He never states that he's writing it for that reason, but why else would he go through all the trouble and risk? He believes that there exists a thing that we might call "the human spirit" and that that spirit is indominable and will ultimately triumph. This is, of course, total nonsense. Winston believes that through the power of will he can (and always will) believe that 2 + 2 = 4. What he learns is that it's totally relative, that he can be made not only to say that 2 + 2 = 5 but to believe it. He totally misunderstands the role of power, power that is executed not "for the good of humanity," but for it's own sake, power exercised without excuse, power that only exists to procreate itself. Next to that, his belief in "the human spirit" pales.
As a generally liberal society, we tend to take Winston's position, that there is an essential humanity that will always be around, that good will prevail over evil (why do we believe this?), essentially that goodness with trump power. I suspect that Orwell is warning us that if we are not vigilant, power will win; this echoes Jefferson's warning about the price of freedom being eternal vigilance.