Final answer:
President Johnson referred to Martin Luther King Jr. as the 'apostle of nonviolence' to highlight King's dedication to peaceful protest and potentially prevent retaliatory violence after his assassination, urging the nation to honor his legacy by continuing to work towards civil rights.
Step-by-step explanation:
President Johnson likely called Dr. Martin Luther King Jr. the “apostle of nonviolence” because King was a lifelong advocate for peaceful resolution to societal issues, which were espoused through the Civil Rights Movement. Dr. King's commitment to nonviolence was deeply rooted in his philosophy, informed by figures like Mohandas Gandhi, Henry David Thoreau, and A. Philip Randolph, and became the cornerstone of the Southern Christian Leadership Conference (SCLC). In the aftermath of King’s assassination, President Johnson's call to “reject the blind violence” that killed King and to continue pursuing civil rights as a tribute to him was perhaps an effort to prevent retaliatory violence and honor King's mission of achieving equality through peaceful means.