In the book, 1984, Chapter 1, why is Winston writing the diary?

When we meet Winston, these rebellious notions have clearly been festering for quite some time. Now, in writing his diary, he is taking the first physical step towards all-out rebellion. In putting his pen to paper, Winston knows he is committing thoughtcrime. He is now a criminal, and knows that his eventual arrest is inevitable. This fatalistic perspective stays with Winston throughout the novel.