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: