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