Heschel counted among his friends other prophets of this period in our history: William Sloane Cotton, Martin Luther King, Jr., Dorothy Day, and others.
He also included among his friends many Christian theologians. In fact, according to his daughter, he said that Reinhold Niebuhr “understood his work better than anyone else.”
In his interfaith dialogues, he avoided conversation on differences; he and his interlocutors explored those religious attitudes which they shared. – http://www.preacherexchange.com/br_abraham_joshua_heschel_essential_writings.htm
“My father felt that the prophetic tradition of Judaism had come alive at Selma,” Susannah Heschel wrote. “He said that King told him it was the greatest day in his life, and my father said that he was reminded at Selma of walking with Hasidic rebbes in Europe. Such was the spiritual atmosphere of the day.
“When he returned, he famously said, ‘For many of us the march from Selma to Montgomery was about protest and prayer. Legs are not lips and walking is not kneeling. And yet our legs uttered songs. Even without words, our march was worship. I felt my legs were praying.’”
See here re a film that has cut out Heschel – http://www.algemeiner.com/2015/01/18/fresh-controversy-hits-selma-daughter-of-rabbi-abraham-joshua-heschel-shocked-by-exclusion-of-her-father-from-film/#