Winston realizes that i'm writing down with big brother in his diary, he has committed a thoughtcrime that will be severely punished when, not if, it is discovered. what is his conclusion, though, as he goes to answer the knock on his door

Winston concludes that the Thought Police have effectively discovered him.

In 1984 Oceania, individuals don't keep individual documentation. Such conduct is viewed as unsafe, as it advances freedom and individual idea. In planning to write in this journal, Winston knows he is carrying out thoughtcrime and along these lines taking a chance with his life.