"the English Evangelicals who abolished the Slave Trade"

Lewis is referring to William Wilberforce (among others), a British Evangelical and author of Practical Christianity (originally titled Practical View of the Prevailing Religious System of Professed Christians, in the Middle and Higher Classes in this Country, Contrasted with Real Christianity), who worked in Parliament for many years to see slavery abolished. He died shortly before the Slavery Abolition Act was passed in 1833.

Wilberforce's story is told in the 2006 film Amazing Grace.