Winston says that the Party will ultimately fail. What does he say will destroy the Party ?
Winston feels that the proles will eventually rebel against the Party and fundamentally believes that the Party cannot maintain a society founded on fear, hatred, and cruelty. Winston realizes that the proles account for 85% of the population and live relatively independent lives. The sheer number of proles is enough to topple the Party instantly if they could ever become aware of their situation. Winston even writes in his diary "If there is hope . . . it lies in the proles" (89). Unfortunately, the proles are completely ignorant and have no desire to organize themselves to challenge the Party.
Later in the novel, O'Brien is torturing Winston and begins to elaborate on what makes the Party superior to all other authoritarian regimes in the past. O'Brien tells Winston that unlike the past regimes, Oceania's society is founded on hatred, which is why Big Brother will remain omnipotent. Winston responds by saying,
"It is impossible to found a civilization on fear and hatred and cruelty. It would never endure." (338)
Essentially, Winston believes that a Party founded on fear, hatred, and cruelty will disintegrate because hatred is more exhausting than love. Overall, Winston believes that a combination of the proles becoming enlightened to their oppressive circumstances and the exhaustive negative energy needed to support Big Brother's authoritarian regime will eventually destroy the Party.
I'm not sure exactly what you're referring to, but I think you're referring to Winston's "faith" in the Proles. This faith is based on some Romantic and idealistic view that the dis/unorganized Proles coudl somehow realize how miserable their situation is and rise up in revolt to overthrow the Party.
This is total nonsense. O'Brien makes it clear that what makes the Party different from all that has come before is its ability to use power as power ... not as a means to reach some "good" end, but rather as an end in itself. According to O'Brien, past dictatorships have made the mistake of not admiting that their final end was power.
This isn't something the Proles can deal with; no one in their society can deal with it.