The primary reason Karl Marx hated religion -- specifically Judaism and Christianity -- was that he regarded it as the "opiate of the masses."
This attitude has permeated all leftist views of religion since Marx: Religion keeps people from making revolutions to materially better their lives. This is the source of the animosity toward religion on the Left (including the "religious Left" -- leftists who change Judaism or Christianity to fit their values).
To understand this, one must understand the essence of Marxism and its offshoots.
The Marxist worldview is based on a materialist understanding of life. In popular jargon, "materialism" means an excessive love of material things. But philosophically, "materialism" means that the only reality is matter, that there is no reality beyond the material world.
That is why, for example, to most leftists it is a great wrong that amid Latin American poverty, the church would build expensive cathedrals. In their view, all that gold and treasure should be spent on the poor. To a person with Judeo-Christian values, on the other hand, while feeding the hungry is a primary value, there are many other values, including the need to feed the soul. Moreover, the fact that many of the world's poor people would prefer having a cathedral to distributing whatever money selling such edifices would provide has disturbed the Left since Marx. To a materialist, the notion that poor people would place non-material concerns over material ones is absurd, if not perverse.
The recent best seller "What's the Matter with Kansas?: How Conservatives Won the Heart of America," by liberal author Thomas Frank, perfectly illustrates this point. The theme of his book is that Americans of a lower economic status who vote Republican do so against their own (economic) interests. When I dialogued with the personable Mr. Frank on my radio show, he seemed incapable of understanding that many millions of Americans consider the Left's attempt to redefine marriage, for example, more important than the alleged economic advantages of voting Democrat.
Because religious people have values that transcend the material, Marx called religion the opiate of the masses: It keeps the masses from making social revolution by keeping them happy with non-material concerns and non-material rewards.
The further left one goes, the more significant social revolution becomes. It does for two reasons:
First, devoid of religious meaning in their lives, for many on the Left, social change -- or as it is known today, "social justice," the term for left-wing social change -- has become their substitute religion and provider of meaning.
Second, given that the only reality is material, any denial of materialism's supremacy disturbs the Left. The true leftist objects to the notion of poorer people leading happy lives. For the heirs of Marx, which is what the Left is, the "good" is not so much a moral, let alone a spiritual, category, as it is an economic one. Hence the left's preoccupation with economic inequality, taxing the rich, etc.
Now, of course, Judeo-Christian values care about material progress. In fact, it was the Bible that gave the idea of progress to the world. It was the Jewish prophets of the Old Testament who first enunciated the divine obligation to care for the poor and the helpless. But such concerns were never the only Judeo-Christian values, and the "poor" in biblical nomenclature were truly destitute, not at all analogous to those classified as "poor" in America. Overwhelmingly, the "poor" of America live in a home with two or more rooms per person and air conditioning, and own a car, refrigerator, stove, clothes washer and dryer and microwave. They have two color televisions, cable or satellite TV reception, a VCR or DVD player and a stereo, and obtain medical care (even without health insurance). Nearly half actually own their own homes.
The Left regards itself as morally elevated because of its preoccupation with materialism. Yet religious Americans who reject materialism are far more likely than left-leaning parents in the same socio-economic condition to sacrifice materially in order to have one parent be a full-time parent. And Judeo-Christian values explain why a religious woman is far more likely to sacrifice materially by giving birth to, rather than aborting, an unplanned child.
Even freedom is a higher value to one who holds Judeo-Christian values. That is why the materialist leftist world so often celebrated and continues to celebrate communist regimes. Those regimes may have rendered their societies prisons, but they (at least in theory) reduced material inequality.
That is why the founders of America, the place where Judeo-Christian values have flourished, inscribed a verse from the Torah, the primary source of Judeo-Christian values, on the Liberty Bell: "Proclaim liberty throughout all the land unto all the inhabitants thereof."
Freedom, too, has no material value.