Complex sentence: During the 1930s, Niebuhr was a prominent leader of the militant faction of the Socialist Party of America, although he disliked die-hard Marxists by calling their beliefs a religion and a thin one at that.
Split into simpler sentences: During the 1930s, Niebuhr was a prominent leader of the militant faction of the Socialist Party of America, although he disliked die-hard Marxists. He described their beliefs as a religion and a thin one at that.