There were several crucial turning points in his life. I can't narrow it down to one.
1. He learned that education was power. As a slave boy, he secretly taught himself to read and write.
2. At 17 his master sent him to Mr. Covey to be broken. Douglass refused to take the abuse any longer, fought back and won.
3. When a free black sailor let him borrow his papers so that he could go north on a train to freedom.
4. After he was a well-known speaker/politican his abolitionist friends legally bought his freedom from his master so that he did not risk being captured and taken back and enslaved in the South.