Now, the interesting part of your question is this: religion and secularism, because of their opposed nature, can only really exist based on the definition of the other. So, to understand the changing nature of the term "secular", one has to understand the changing nature of the term "religion" as well.

The words "religion" and "secular", as with many dichotomies, exist only because of their relation to each other. The idea of "religion" as its sort of own entity really starts to come around in the 1500's. As religion began to be defined as its own sphere away from the practice of civil activities, there had to be a denotation for the non-religious world which is where the word "secular" comes in.

Everything else then starts to become exceedingly complicated. Even after hundreds of years of discussion, scholars have yet to come to a solidified definition on what exactly "religion" is. Is it belief? Is it action? Is it based off of texts? Are gods a requirement?

A relatively recent attempt at defining religion by Mandaville and James is "a relatively-bounded system of beliefs, symbols and practices that addresses the nature of existence, and in which communion with others and Otherness is lived as if it both takes in and spiritually transcends socially-grounded ontologies of time, space, embodiment and knowing" (Mandaville and James, xii-xiii). But could this definition not also apply to experiencing a song you love or "secular" philosophies that appeal to an idealized concept like justice or freedom? Also, to what extent of relativity is acceptable within this definition of a "relatively-bounded system"?

In contrast to that, the idea of "secular" typically is taken to mean "not religious", but we use "religious" as a descriptor to define the way in which we engage with secular activities. How often have we heard someone say something like, "I follow (sport/sports team) religiously"?

An interesting example to think about in relation to this topic is the concept of "civil religion". In the modern age, people often tend to think of civil religion as aspects of the nation/state compared with religious concepts. However, Rousseau's original idea of civil religion was the state actually upholding religious practice.

I've tried to keep this response brief and to the point because there have been many gallons of ink utilized to debate this issue and I didn't want to go into a bunch of tangents. Religion and Secularism have never truly been mutually exclusive things, despite the idea of them being so. They are constantly being re-defined and shifted in relation to each other as cultural and social thought changes.

Recommended Reading

• Asad, Talal. Formations of the Secular: Christianity, Islam, Modernity. Stanford: Stanford University Press, 2003.
• Bellah, Robert. "Civil Religion in America." Daedalus, Journal of the American Academy of Arts and Sciences 96 (1967): 1-21.
• Eliade, Mircea. The Sacred and the Profane: The Nature of Religion. New York: Harcourt, Inc., 1959.
• Geertz, Clifford. "Religion as a Cultural System," in The Interpretation of Cultures. New York: Basic Books, 1977.
• Goossaert, Vincent and David Palmer. The Religious Question in Modern China. Chicago: The University of Chicago Press, 2011.
• James, Paul and Peter Mandaville. Globalization and Culture, Vol. 2: Globalizing Religions. London: Sage Publicatiosn, 2010.
• Masuzawa, Tomoko. The Invention of World Religions: Or, How European Universalism was Preserved in the Language of Pluralism. Chicago: The University of Chicago Press, 2005.
• Smith, J.Z. Relating Religion: Essays in the Study of Religion. Chicago: The University of Chicago Press, 2004.