How does Frederick Douglass establish credibility with a white northern audience?

Margaret Fuller, a well known book reviewer and literary critic praised Douglass and his work. She wrote, "we have never read [a narrative] more simple, true, coherent, and warm with genuine feeling."[Margaret Fuller, critic: writings from the New-York Tribune, 1844-1846, Volume 1] She also described the preface of the narrative, in which two white men wrote on Douglass' behalf. This aided in establishing his credibility with a white northern audience.