# everyone whose name was not written ... in the Book of Life

"those whose names the Lamb did not write ... in The Book of Life" or "those whose names were not ... in the Book of Life"

# from the foundation of the world

"from the time that God created the world"

# the Lamb

A "lamb" is a young sheep. Here is it used symbolically to refer to Christ.

# who had been slaughtered

"whom the people slaughtered"