In 1963, Steve Schapiro, then 28, was on assignment for LIFE magazine, photographing prominent civil rights activists, from James Baldwin to Fannie Lou Hamer. One day, while following Jerome Smith, a participant in the Freedom Rides that raised awareness of interstate bus segregation, he went to Clarksdale, Miss., to document one of the many training sessions that were taking place in church basements across the South. In those meetings, volunteers studied how to react to the racism they would encounter in their work. That day in Clarksdale, as Schapiro watched a line of ministers file into the church, he noticed among the group another well-known Freedom Rider, in a tie and button-down shirt: John Lewis. He asked Lewis if he could take his photo, and the young man agreed.