One major argument that apologists for religion like to make against proponents of secularism, humanism and religion is to equate all opponents of religion with Communism and the numerous crimes against humanity perpetuated by such monsters as Lenin, Stalin, Mao, Ho Chi Minh, Pol Pot and Fidel Castro. The best argument against these people of faith is a simple one, far from being a humanist or rationalist belief system, Communism was and is a religion.
Like all religions, Communism is irrational, dogmatic and based on faith rather than science. Just like Christianity and Islam, Communism had its Holy Books which were treated as Holy Scripture, namely the writings of Lenin, Mao, Marx and others--all of which were far from scientific. Karl Marx, who was treated by Communists as a genius, was actually a small-time journalist whose writings are a collection of prejudices, generalizations and editorializing. Marx held and promoted some beliefs which were later disproved by science, for example Marx taught that many human characteristics we now know to be inherited through genetics were caused by environmental factors. When scientists in 1930s Russia pointed this fact out, Stalin reacted by throwing the scientists into the gulag just like the Church imprisoned Galileo. Just like fundamentalist Christians who promote creation science, Stalin (himself the recipient of an "education" in a Christian seminary) backed a charlatan named Lysenko who came up with a completely false science of genetics that fit squarely with Communist dogma and then banned the teaching of genetics because it contradicted Communist dogma.
As with Christianity and Islam, Communism attracted followers by promising a pie-in-the-sky heaven to the faithful. The difference being that the Communist heaven would be sometime in the future when all people would be happy and equal under Communism rather than after death. This magical future was conveniently pushed farther and farther into the future so that Communist leaders could "explain" to the average people impoverished by their wonderful system why they hadn't yet achieved utopia. It might also be pointed out that the Communists never actually said exactly how this utopia would be created--just as Christians and Moslems can present no evidence of life after death.
Like most religions, Communism operated on irrational faith; people in Communist countries had to have absolute faith in the Communist system and its leaders. Thinking for oneself was strictly verboten in Stalin's Russia, Mao's China, and Ho's Vietnam. Those who questioned Communism and its leaders were treated as heretics by the Communist state.
Far from being an example of the evils that occur when religion is removed in society, Communism is a perfect example of the excesses and horrors that result when religion is allowed to take over a society. The Communist Party acted just like the church had in Medieval Europe....