Frederick Douglass, one of the most significant abolitionist speakers, greatly affected American social policies by writing significant biographies of his life as a slave, advocating women’s rights, and arguing for the establishment of colored troops in the Union army
I think it's a great example of an intro thesis sentence, especially for AP US History if that's the class you're in. I think you could use a clarifying sentence, to hammer the message home. Something that argues the same point in a slightly different way sdo the reader knows without doubt what the thesis is.
This is well crafted and descriptive of who Douglass was, but it seems more like an introductory paragraph for an encyclopedia entry than a thesis. When writing a thesis, you are making an argument about "how and why" Douglass was a successful abolitionist. The "what, when and where" are important but are background facts; the setting of your thesis - the intro you've written already.
Your thesis needs to be more direct and specific. So, the question following this introduction would be the "why and how" Douglass carried out his activism. For example, if you talk about his public speaking and writing, pick out a theme from these; and how he utilized this theme to get his message across. You might follow this intro with a focus on education and literacy, which he believed would be the liberating factor. This could be a thesis on Douglass' correlation with freedom (including freedom of thought) and literacy. Ex. Douglass achieved success in his activism by stressing the role of education and literacy in overcoming oppression. Or, if you focus more on a theme like morality and religious arguments against slavery, you'd follow your introduction with a statement like; "One of Douglass' strategies was to show that slavery was a moral and religious violation, as well as a violation of the Declaration of Independence (and this could later segue into a sub-theme of government's role.
So, great intro; just specify a "why" and/or a "how" Douglass did what he did.