Mark Whitaker is the author of “Smoketown: The Unknown Story of the Other Great Black Renaissance.”

In the summer of 1947, at the age of 18, Martin Luther King Jr. decided to join the family business and become a preacher. After spending three years at Morehouse College socializing more than he studied and flirting with becoming a lawyer, he delivered the news to his father, the pastor of the Ebenezer Baptist Church in Atlanta. Overjoyed, “Daddy” King had his son ordained and installed him as assistant pastor. At that point, young Martin could have stayed in Atlanta and settled into the comfortable life of a minister destined to inherit his father’s pulpit. Instead, he broke more news to his parents during his senior year: He planned to go to a seminary to pursue a bachelor of divinity degree. And not just any seminary: one up North, with a liberal, all-white faculty that prided itself on demythologizing the biblical figures of Moses and Jesus and introducing students to critical thinkers from Saint Augustine to Kant.

King’s three years at the Crozer Theological Seminary, south of Philadelphia, marked an important turning point in his life and are well worth the exclusive focus they get in this compact, readable and well-researched book. A teacher and freelance journalist with a long-time interest in King, Patrick Parr has mined the papers at the King Center in Atlanta and at Stanford University, as well as archived interviews conducted by Taylor Branch and David Garrow, the authors of the best-known books about King, which devote relatively few pages to the Crozer story. Parr tracked down several classmates of King’s, who shared personal memories and took him on a tour of what’s left of the campus, which was closed in 1970. Tantalizingly, he also interviewed a white woman with whom King became briefly involved while at Crozer: Betty Moitz, then the 20-year-old daughter of the seminary cook.

Parr’s most interesting revelations trace King’s growth as a preacher and public speaker. Before he arrived at Crozer, his models in that area were his forceful but unpolished father, a smoother rival from another Atlanta church and the distinguished president of Morehouse, Benjamin Mays. At Crozer, King studied preaching every semester with Professor Robert Keighton, a lover of English literature who introduced him to classic sermon forms (with names like “three-points-and-a-poem” and “jewel,” in which a theme is examined from every angle), as well as to orating for the radio. But King also received feedback from the Rev. J. Pius Barbour, an African American pastor at a nearby church who invited black seminary students to his house for home-cooked meals and tough-love critiques, urging them not to lose touch with the call-and-response tradition of their churches while absorbing “the White Man’s intellectualism.” By the end of his first year at Crozer, King was giving guest sermons at nearby churches and was acknowledged as the finest preacher among his peers, a reputation that later won him election as class president.

The “love story” with Betty Moitz, teased throughout the book and in a foreword by Garrow, turns out to be tamer than advertised. From chats in the cafeteria, King’s friendship with the cook’s daughter, an interior-design student with brunette bangs, graduated to movie dates and kissing on the Crozer campus. But when King mused to Barbour and a fellow black student about the possibility of interracial marriage, they warned about the impact on his future standing in the black community. King immediately “cooled it,” as the classmate put it. And how serious King ever was about Moitz is unclear, since Parr mentions three black women he was courting around the same time. Reached by correspondence and then in one interview, Moitz, in her late 80s, described the relationship as two people “madly in love,” then simply as “short and sweet.” In the end, she concluded what most of King’s girlfriends did at the time: that his priority was his career and preparing himself to “return South to help.”

Presumably, readers will come to a book called “The Seminarian” expecting to discover how three years in divinity school influenced King’s religious and political ideas. On this score, Parr offers course catalog descriptions and some vivid stories, but not much in-depth reflection. He relates unpleasant episodes at local diners that stoked King’s anger at Northern racism. He identifies the experience that fired King’s interest in Mahatma Gandhi: when King went into Philadelphia to hear a speech by Mordecai Wyatt Johnson, the president of Howard University, who had just returned from a trip to India. Parr also documents several cases of King’s absorbing new thinkers by lifting their writing verbatim into his papers, a plagiarism habit that went unpunished at Crozer but later caused controversy over his Boston University doctoral dissertation. Yet apart from a paragraph about pool room bull sessions with the professor who introduced King to Reinhold Niebuhr, Parr gives little sense of how shaken and influenced King was by reading the “realist” theologian in his last year at Crozer.

On the 50th anniversary of his assassination this month, a new HBO documentary and numerous commentaries have reminded us that King was no Pollyannaish integrationist but a fierce fighter for justice in all its forms, economic as well as legal. In his 1958 book, “Stride Toward Freedom,” King discussed how wrestling with Niebuhr’s pessimism about the limits of pacifism and the intransigence of collective evil forced him to rethink nonviolence as an instrument — not to change people’s hearts, but to confront and expose violently unjust institutions.

As King’s friend Rabbi Abraham Joshua Heschel later identified, King’s vision of himself as a “drum major for justice” also had much in common with the Old Testament prophets, whom he studied in depth at Crozer.

Here, too, Parr offers little analysis, but he does leave readers with a memorable image. As classmates passed King’s room in “Old Main,” the imposing central building on the Crozer campus, they often heard him rehearsing the delivery of the verse from the Book of Amos that he would invoke again and again over the next two decades, up to the impassioned “Mountaintop” speech he gave the night before his death. “Let justice run down like water,” King recited, “and righteousness like a mighty stream!”

The Seminarian

Martin Luther King Jr. Comes of Age

By Patrick Parr

Chicago Review Press. 286 pp. $26.99