This is a graph of the frequency of occurrences of the word "sin" (case-sensitive) in a 5% sample of all American works in English in Google Books.
The volatile line in the 17th and 18th centuries is partly because there is a relatively small number of works available from that era. It's interesting how the frequency of "sin" in written literature shoots up not at, but a bit after both the Great Awakening (ca. 1740) and the Second Great Awakening (ca. 1800). I wonder how much of the decline of the appearance of "sin" through the 19th and early 20th centuries is due to the explosion of Christian publishing in antebellum America before secular publishing gradually caught up. It's also interesting to note the small bulge during the post-WWII religious revival (during which time "under God" was added to the pledge of allegiance, for example) which tapered off in the 1970s to rebound slightly and remain basically flat since.
A few questions for the comment thread. (1) How does this graph reinforce or challenge what you've believed about American religious history? (2) What does this graph tell us about the era we're living in? (3) What can this graph teach us about how we can proclaim the gospel effectively?