One of the fastest ways to really piss people off is to inform that that their favored view of history is wrong. We use history as a mythology to explain (or proclaim) who we are, and to make claims as to what our proper course of action should be.
If you don't believe me, try telling a fundamentalist Christian that many of the Founding Fathers of the United States were not Christian, or try telling a politically-motivated secularist that a number of them were, in fact, Christian. Sit back and watch the orgy of invective ensue. One side will scream that the founders were all Christians (usually implying that the Christianity was of a decidedly 20th/21st century form) and that they created the U.S.A. as a "Christian Nation" (thus proving that these people have little knowledge of the history of the nation, and even less knowledge of the Constitution that is the basis of its law), while the other side will insist that the founders were deists, not Christians, and that they intentionally created a secular nation (the simple fact of the matter is that the founders were a religiously diverse group, though the secular nation part is actually backed by the Constitution - which only mentions religion only twice, both times forbidding government interference or endorsement - read it here and here if you don't believe me, look for the part that makes the U.S. a Christian nation - it isn't there).
Why so much grief over what these people believed over two centuries ago?
Because the argument isn't about the past, it's about the present. People who want to be able to force their religion on others cling to the fiction of a group of founders who were entirely Christian and who wished for a Christian nation because this lets them feel that history is on their side, and that they are simply fulfilling our forefather's will. By contrast, those who oppose them are also clinging to a fiction in an attempt to discredit their opponents and claim that their opponents are "dangerously un-American."
The truth of the matter is rather more complicated. The "Founding Fathers" itself is a vague term, and may refer to all of the signers of the Declaration of Independence as well as the delegates to the Constitutional Convention (a list of whom can be found here), or it may be used to refer to a sub-set, depending on the intentions of the person identifying the Founding Fathers. The entire group is large and composed of people of a wide variety of political, religious, and social views. It includes both Christians and deists, as well as atheists and people who defy religious description. Members of the group wrote a good deal about their views on the role of religion in government, and someone who wants to cherry-pick can easily find something that supports their views, provided that they ignore everything else that is relevant to the discussion. From a legal/government standpoint, all of that pales in comparison to the fact that these people all eventually agreed to a Constitution, intended as the law of the land, that separated the government and religion.
From a social mythology standpoint, however, the actual fact of what is written in the Constitution matters little, what is more important is the question of the beliefs of the Founding Fathers. And so, here we are, with two competing mythologies, one which portrays the Founding Fathers as entirely Deist, the other of which portrays them as entirely Christian. Both are false, but that doesn't remove the power of these stories amongst those ready to accept them. Those who cling to the Deist myth are afraid that acknowledgement of Christian founders will result in a theocratic takeover, and a loss of legitimacy to secularist positions. Meanwhile, those who buy the myth of a Christian founding are afraid that acknowledgement of the rather more complex founding, and the decidedly secular nature of the Constitution will erode their own religious rights. And so these two sides continue to fight, never having any hope of conclusion or victory because both are too blinded by the politics of the present to honestly evaluate the past.
While this sort of thing is most visible in church/state separation fights, it is common in many other public historical narratives. I have written before about the White and Black legends of Spanish colonization of the Americas. Both the white Legend of Spanish benevolence and the Black Legend of Spanish malfeasance are rooted in modern politics and mores, rather than in a fair accounting of the historical record. The same can be said for the competing narratives regarding the spread of Christianity/quashing of Paganism in the Roman Empire, the heroism or villainy of figures such as Christopher Columbus, the glory or evil of Manifest Destiny, even questions as to whether or not Keynesian economics helped lift the U.S. out of the Great Depression, etc. etc.
History is a vast and very complicated mosaic. There are many reasons to study it - it's interesting for one thing, but it can also provide clues as to what our problems are and how to solve or avoid them. History is also a source for ideological mythology. This is a fact that was well understood by the early historians - the Roman historian Livy, for example, made no bones about the fact that he was more interested in telling a story that glorified Rome than in telling one that was accurate. Livy understood the power of a mythological past, and we should probably be grateful that, unlike most modern spinners of myths, he was at least open about the fact that he was doing this, allowing us to more easily take what he said with a grain of salt.
Likewise, governments have long understood the value of history as myth. It's a rare nation that hasn't endorsed an official account of the past, which is never an accurate account. Totalitarian governments even go so far as to outlaw non-official versions (there are numerous accounts of historian and archaeologists running afoul of Stalin's government, and Hitler was just as certain to go after academics as to go after active dissidents, just to give two examples), indeed, the fact that academics are able to publish unpopular accounts of the past is not only evidence that they are probably giving accurate accounts (the truth is rarely as favorable or condemning as most people want it to be), but also that our society truly is a free one.
Most responsible researchers can not get away from their own perspectives, but responsible researchers are aware of that and fill their books and articles with qualifiers and references to aid the reader in evaluating the arguments. When you are fed a historical narrative by members of your social group, or leaders of your political party, or political activists (whether you're a fan of the Green Party or of the Tea Party), or clergy, or industry leaders...well, you get the point...by very skeptical of what they are telling you. Odds are that their account of the past is more about what they want in the present than what truly occurred.