The view is put forwards that the atrocities committed in the twentieth century, particularly in Germany and the Union of Soviet Socialist Republics, were due to widespread acceptance of the Stoic doctrine that man was the measure of all things, a doctrine made popular by scholars of the Renaissance and the Enlightenment. The move from belief in original sin, a Supreme Being, and the imperfection of man, to man as essentially good and self-perfectible, directed by his own reason, is discussed. The effects of Communism and National Socialism, and the rejection of traditional morality in favor of secular humanism are highlighted. It is concluded that this move led to the worst atrocities known to man. A return to the law of nature, a code of morality and the moral compass of religion which has endured the test of time is advocated.