As other educators have noted, this sentence appears in a dream to Winston and is uttered by O'Brien, a fellow party member. When it first appears, in Part One, Chapter Two, it represents Winston's internal desire to rebel against the party's control. But Winston's thoughts on this matter are suddenly interrupted by a "trumpet call" from the telescreen. This interruption forces Winston to abandon his thoughts and, on a deeper level, to realise that resistance against the party is futile.
But, over time, Winston's sense of internal rebellion heightens to a point that he can no longer ignore it. In Part Two, Chapter Eight, for example, he goes to O'Brien's apartment and we hear the phrase repeated again. This is a critical moment for Winston because he has taken his internal rebellion to a new level: he has made himself known to O'Brien and is about to receive a copy of Goldstein's book. The fact that Winston is the one who utters this phrase is indicative of his optimism. Winston thinks that O'Brien has recognised "this allusion" but it is doubtful that he has. It is more likely that he played along to encourage Winston in his anti-party activities.
Ironically, O'Brien is not the man Winston believes him to be; he is a member of the Thought Police and "the place in which there is no darkness," is, in fact, the Ministry of Love, and is a symbol of Winston's torture and reintegration into society. Just like the party's ironic slogans, like War is Peace, irony is at the very heart of 1984 and defines the course of Winston's life.