From the book:
America's Revolutionary Mind – A Moral History of the American Revolution and the Declaration That Defined It, by C. Bradley Thomspon
(see this review; buy it here, here, or here), on p. 322:
To modern historians, the notion of a spirit of liberty might seem like little more than flowery rhetoric. To eighteenth-century Americans, however, it meant something specific and essential. The word "spirit" as used in the phrase signifies both an idea and action, [...] [T]he spirit of liberty means to "assert and maintain the cause of liberty." The phrase "spirit of liberty" united theory and practice for American revolutionaries; it implied an action in defense of a principle; it was characterized by certain virtues in the defense of liberty. [...] "We know our rights, liberties, and privileges, and likewise men are determined to sacrifice all things else for their preservation." [...] The spirit of liberty, then, is a sense of life defined by independence in the fullest sense of the term.
The history and symbolism surrounding it and depicted in it provide a good overview and introduction to humanity's rise from savagery and barbarism to rationality and civilization – what some people short-sightedly refer to as "Western" culture.