In 1904 he converted to Catholicism, and later went on to be the first Roman Catholic co-chairman of the National Conference of Christians and Jews alongside Everett Clinchy and Roger Strauss, serving as co-chairman 1928 to 1946 despite a 1928 papal encyclical which explicitly prohibited such ecumenical interactions. He was chairman of Columbia's History department several times. After World War I, he joined with Peter Guilday in establishing the American Catholic Historical Association and became its first secretary. Its goal was to promote Catholic history and to integrate Catholic scholars into the wider academic world.