Why does emerson most likely devote so much time to john brown's family history at the beginning of his speech?
to diagnose the root causes that led to his arrest
to portray him as a unique and exceptional individual
to show that he was in many ways no different from the audience
to enumerate how many people have been deeply affected by his arrest

to show that he was in many ways no different from the audience.

Emerson's main purpose was to increase sympathy for the plight of John Brown. The way in which he encouraged this was by giving information about Brown's private life, such as information on his wife and children. He believed this would show that Brown was not a criminal or a mythical figure. Instead, he was just a common man who had ideals and wanted to fight for them.

taking into account that a theme is the central topic a text treats and can be developed through a story, play, poem etc. the following are themes that can be further developed:  

my best friend jeffrey has many unusual interests. (the story could be about each of the unususal interests)

once a man gives his soul over to evil, he can never go back. (this could be a saying or a proverb)

the three sheepdogs lay around the yard all day. (it could be developed detailing how each of these sheepdogs lay around)

a priest's collar has an unusual effect upon people. (it could be developed detailing the different effect upon people)

innocence is a powerful force in a world full of corruption. (it could be developed detailing how is a world full of corruption)

king lear is a play shakespeare wrote later. (it is just a sentence and doesnt give any idea or topic to be discussed in details.

evil often wears the disguise of innocence. (this could be a saying or a proverb)