What does Woolf say about freedom of thought at the end of the piece?

Woolf says earlier that Englishwomen have had less intellectual freedom—in other words, freedom of thought—than the Athenian slaves. She goes on to say at the end that

if we have the habit of freedom and the courage to write exactly what we think[...]then the opportunity will come.

That opportunity is the opportunity to write great literature.
Woolf argues that it is deeply liberating for women to think for themselves and not have their thoughts dictated by men. She contends that it is very difficult for women to separate their ideas and desires from men's ideas and desires when they don't have economic independence and privacy. In other words, if not agreeing with the men in your life means you could be thrown on the streets with no reasonable way to earn an income, you will tend to suppress your own ideas and imagination.
She looks at the London statues put up to commemorate victories at war and because she has the economic resources to think for herself, she can separate herself from these monuments and realize they commemorate men, not men and women. Freedom of thought allows her to imagine, too, a "hundred years" later women having economic opportunities equal to men, and "Shakespeare's sister" producing plays as good as his. Her main point is that money matters to the creative life.