Many of the Rev. Martin Luther King Jr.'s most formative writings and sermons -- some dating to when King was a precocious 19-year-old seminary student in 1948 -- languished for decades in a battered cardboard box.

A decade before her death in 2006, King's widow, Coretta Scott King, flew to San Francisco to ask Stanford Professor Clayborne Carson to examine and write about the box's contents.

The texts, which illuminate the theological foundations that America's most celebrated social activist would repeatedly return to, are revealed in a book to be released today -- Martin Luther King Jr. Day -- by Stanford University's King Papers Project.

The collection includes documents from 1948 to 1963 -- the years covered by the book -- and "gets us closer to King's true identity" because they shed new light on how he viewed the Bible, Carson said.

"King used to say, 'People think of me as a civil rights leader, but fundamentally, I'm a Baptist preacher,' " said Carson, editor of "Advocate of the Social Gospel," which is based on the newly disclosed writings and is the sixth book produced by the King Papers Project.

The texts are triggering a discussion about how much King's rejection of a literal reading of the Bible shaped his social activism.