This has often been attributed to de Tocqueville's Democracy in America, but erroneously, according to "The Tocqueville Fraud" http://www.weeklystandard.com/print/the-tocqueville-fraud/article/8100 in The Weekly Standard (13 November 1995). This quote dates back to at least 1922 (Herald and Presbyter, September 6, 1922, p. 8 http://books.google.com/books?id=3sYpAAAAYAAJ&pg=RA3-PT21&vq=%22I+sought+for+the+greatness+and+genius+of+America+in+her+commodious%22&source=gbs_search_r&cad=0_1)
There's an earlier variant, without the memorable ending, that dates back to at least 1886:
I went at your bidding, and passed along their thoroughfares of trade. I ascended their mountains and went down their valleys. I visited their manufactories, their commercial markets, and emporiums of trade. I entered their judicial courts and legislative halls. But I sought everywhere in vain for the secret of their success, until I entered the church. It was there, as I listened to the soul-equalizing and soul-elevating principles of the Gospel of Christ, as they fell from Sabbath to Sabbath upon the masses of the people, that I learned why America was great and free, and why France was a slave.
Empty Pews & Selections from Other Sermons on Timely Topics, Madison Clinton Peters; Zeising, 1886, p. 35 http://books.google.com/books?id=f54PAAAAIAAJ&pg=PA35&dq=de+tochneville&ei=w1YCSbS3JoTkygS2g_mvDQ
Misattributed
