Marius began life on a farm in East Tennessee, evolved into a figure of 1960s campus political activism, and became a respected Reformation historian on the Harvard faculty.
His father was an immigrant from Greece who earned a chemical engineering degree in Belgium before settling in the United States, where he managed the foundry at the Lenoir Car Works of the Southern Railway.
He was particularly affected by Stace's essay Man Against Darkness, which includes the statement that: His novel An Affair of Honor (2001) features a protagonist, Charles Alexander, who like Marius becomes caught between the traditional morality of his upbringing and the freethinking he encounters at the University of Tennessee and in W.T.
Attending college classes in the morning, he worked in the afternoons as a reporter for the Lenoir City News, writing a column called "Rambling with Richard".
He took a year off, spending 1956–57 in Europe as a Rotary Fellow in history at the University of Strasbourg, then returned to another Southern Baptist Theological Seminary in Louisville, Kentucky, from which he graduated with a B.D.
After graduating from Yale, Marius taught history at Gettysburg College from 1962 to 1964, before returning to his home state to take a position on the faculty of the University of Tennessee in Knoxville.
Most notably, shortly after the Kent State shootings, he co-organized a protest at a 1970 Billy Graham evangelistic crusade rally in the university's football stadium at which President Richard Nixon was an announced, invited guest.
Marius' sometimes provocative statements and political efforts, which clashed with the prevailing view in the conservative state of Tennessee, led to threats against him and his family.
The following year, he married Lanier Smythe, an art historian who later became chair of humanities at Boston's Suffolk University; they had a son named John.
He spent the last twenty years of his life at Harvard, producing most of his major work there, including his biographies of Thomas More and Martin Luther and his final two novels.
He coached the students charged with delivering annual commencement addresses each year and helped Harvard's presidents develop their graduation speeches.
He served as a faculty advisor to the Signet Society, a creative arts club, and he and his wife spent a semester during the 1996–97 academic year as acting masters of Adams House, an undergraduate residence hall.
After being diagnosed with pancreatic cancer in 1998, Marius retired from teaching in order to focus on completing his final novel, An Affair of Honor, amid the rigors of chemotherapy.
His ashes were buried below Author's Ridge in Sleepy Hollow Cemetery in Concord, Massachusetts, near the graves of Ralph Waldo Emerson, Henry David Thoreau, Nathaniel Hawthorne, and Louisa May Alcott.
In 1995, Vice President Al Gore personally offered Marius a White House speechwriting position heading into the 1996 presidential campaign.
Marius had previously written, without pay, several speeches for his fellow Tennessee native, including a 1993 Madison Square Garden oration for the fiftieth anniversary of the Warsaw Ghetto Uprising and parts of Gore's 1994 Harvard commencement address attacking the "culture of cynicism".
Marius accepted the offer to join the White House, took an eighteen-month leave of absence from Harvard, rented out his home, and prepared to move to Washington, DC.
But Gore rescinded the offer after New Republic editor-in-chief and part-time Harvard social studies lecturer Martin Peretz pressured the vice president to reverse Marius' hiring.
Peretz told The Washington Post: Marius allowed that his Gestapo–Shin Bet comparison may have been "a little bit extreme", but he refused to disavow it, insisting that he was criticizing only the harsh tactics of the secret police and otherwise supported the state of Israel.
One of the pre-eminent Reformation scholars of his generation, Marius' two major scholarly works were biographies of Thomas More (1983), the English lawyer, Utopia writer, and politician who persecuted Protestants before being beheaded for refusing to accept Henry VIII's break with Catholicism, and of Martin Luther (1999), the monk whose criticism of the Catholic Church inspired the Protestant Reformation.
Both books were also controversial because they stripped their subjects of the sanctity ascribed to them by admirers, instead presenting them as human beings struggling with their beliefs, fears, ambitions, strengths, and weaknesses.
In the final year of his life, Marius traded bitter and sometimes personal academic attacks with Heiko Oberman, a rival Reformation historian at the University of Arizona, who had written his own biography of Luther.
He advised making a rough outline before beginning to write and getting to the point quickly by setting up in the opening paragraph tensions that will be resolved by the end.