But why? Well...a good bit of it is political and some philosophical. Since the 70's the so-called Religious Right has become a formidable political bloc and by its philosophical design, a natural enemy to the Left's shallow interpretations of the world. Mainly on two points, abortion and more recently, gay marriage. Therefore for these reasons Christianity is not only to be opposed...but destroyed, at any opportunity.
The greatest absurdity that the modern American Left suffers from is that their progressive influences, in part, stem from the social gospel movement of the late 19th century. People such as Walter Rauschenbusch and Washington Gladden--Protestant ministers --advocated for such items as the unionization of American workers, minimum wages, child labor laws, etc. Even the stalwart Progressive hero, William Jennings Bryan was a devout Presbyter whose religiosity greatly influenced his social outlook versus some of the modernist progressives of his day.
Another paradoxical point to ponder is the Left's collective thought that Christianity was causative in the rise of the slavery epidemic which infected the American South. When in fact, opposition to slavery and the Abolitionist movement had its roots to the "Second Great Awakening" (SGA). In short many people in the late 1700's America, contrary to popular belief, rarely attended church. They instead replaced traditional worship with revivals which served as not only ritual devotions but "social meetings" between neighbors.
Subsequently,these revival movements were causal in the formation of Christian reform groups whom opposed slavery on theological and moral grounds. See the American and Foreign Anti-Slavery Society. Now...this isn't to say there weren't others who opposed slavery on more than just Christian values, however, there is no doubt that religiosity played a pertinent role for many abolitionists and anti-slavery movements of the early and mid 1800's.
Finally, revisionism is a hallmark of the the modern American Left. This is induced by the Liberal's world of anxiety in which they perceive their political legitimacy under constant threat from any oppositional thought they face.Their media and marketing constructs are selling a bill of goods which is sadly distorted and designed to produce an outcome that will ensure their authority to which they feel entitled.