The inscription on the bell, "Proclaim Liberty throughout the Land.," points to the reason that the bell became a symbol of the antislavery movement in the 1830s.
As is often the case, however, the full verse from Leviticus 25:10 presents a fuller picture. The verse is: "And ye shall hallow the fiftieth year, and proclaim liberty throughout all the land, unto all the inhabitants thereof: it shall be a jubilee unto you: and ye shall return every man unto his possession, and ye shall return every man unto his family."
The 50th year was to commemorate William Penn's 1701 Charter of Privileges, which was an important model for representative government and religious toleration.
The reason the verse was chosen may reflect Quaker opposition to slavery but may also have reflected growing concern over land grabs on the frontier causing increasing friction with the Indian nations.
In any event, in addition to being an icon of the struggle for independence, the Liberty Bell speaks to humankind's striving to make amends and restore relationships. It is essential that we know and understand our history in order to make informed choices now and in the future, and in this light, slavery is just one of the stories to be told.