What did frederick douglass write to prove to his critics that he had indeed been a slave? why did his critics doubt he had been a slave?

Frederick Douglass wrote autobiography “Narrative of the Life of Fredrick Douglass, an American slave”. The critics doubted that he was a slave because he was able to read and write and he was also smart and intelligent

Explanation:

Fredrick was a slave who escaped and became a well renowned author activist and a speaker.  He had written a dozen of books delivered a lot of noteworthy speeches though he had received a very little formal education. He was also the lawyer for the women’s right and he fought for the right of women to vote.

He was a prominent leader of the abolitionist party and he dedicated most of his life for the upliftment of people from slavery. He was talented and intelligent so the critics refuse to accept that he was from the slave society

what are the main ideas?