George Marsden, a history professor at the University of Notre Dame and author of several books on religion in American history, gave the following lecture at a conference for the Organization of American Historians in 2007. In the video, Marsden points out the role that Protestantism had in shaping American religious history. In addition, Marsden counters the "Christian Nation" assertion by pointing to the religious plurality that Protestantism brought to the shores of the "New World." Marsden is one of the most respected historians on American religious history, and is himself a practicing Christian. Along with historians like Mark Noll and Nathan Hatch, Marsden has labored to shed light on the origins and influence of the "Christian Nation" argument.
The video is short, but is a nice "appetizer." For a more in depth look into this topic try Marsden's books, Fundamentalism and American Culture" and Understanding Fundamentalism and Evangelicalism.