Answer:
In my opinion the statement could be the one what we have learned from his autobiography.
He called both the owner and slave a victim of the system. We all are bound to the systems, to the expectation of the others. Even after all those years we still don't have such kind of freedom in which we are free to do anything whatever we want to do because we can not use our free will due to the universal pressure, sometime religious pressure, society pressure or sometimes our own fear what will people say.
By his life we can learn nothing is impossible if we have determination to achieve it, if we got the idea, willingness, dream to have it and then the struggle to have it.
He was beaten by white people but nothing can stop him to achieve his freedom. Everything has a price to pay. But unless you don't feel anything wrong from your heart or head nothing can help you to stop it. but first one needs to explore the truth within himself, around himself.
So I think the only best statements can conclude thesis better is;
without struggle, there can be no progress
I prayed for my freedom for twenty years, but received no answer until I prayed with my legs.
I don't know I was a slave until I found out I couldn't do things that I wanted.
Explanation:
It's very hard to select one statement to conclude it but these statements overall cover the main theme of the autobiography.
Frederick Douglass emphasis not on the freedom but the struggles and bearings of the slave what they face for freedom.
