Complex sentence: Sometimes slaves established their own Sabbath schools to talk about the Scriptures and, in some cases, those who were literate taught others to read, as Frederick Douglass did while still enslaved as a young man in Maryland.
Split into simpler sentences: Sometimes slaves established their own Sabbath schools to talk about the Scriptures. Slaves who were literate tried to teach others to read, as Frederick Douglass did while still enslaved as a young man in Maryland.