Quite a number of excellent scholars, including my friends Tisa Wenger and Winni Sullivan, have essays in this very important new volume to call to your attention: Comparative Secularisms in a Global Age (Macmillan, 2010). Tisa's piece, which I was privileged to read in draft form, is entitled "God and the Constitution: Towards a History of American Secularisms"; in it she discusses a number of nineteenth-century groups who fought to establish "secularism" as a viable philosophy in a very evangelical United States -- including atheists as well as people who effectively could be described as "Protestant but still secularists." What becomes clear, at least from the portions of the book I looked at, is that defining "secularism" is about as tricky as defining "religion," and that, like juxtaposing slavery and freedom, the one cannot be defined without the other.
The book's description:
Comparative Secularisms in a Global Age explores the history and politics of secularism and the public role of religion in France, India, Turkey, and the United States. It interprets the varieties of secularism as a series of evolving and contested processes of defining and remaking religion, rather than a static solution to the challenges posed by religious and political difference. It features essays from leading scholars from across disciplines, secular and religious traditions, and regional expertise. The volume illustrates a new approach to the hotly contested relation between political authority and religious tradition.