“In the end, the state of the Union comes down to the character of the people. I sought for the greatness and genius of America in her commodious harbors and her ample rivers, and it was not there. In the fertile fields and boundless prairies, and it was not there. In her rich mines and her vast world commerce, and it was not there. Not until I went into the churches of America and heard her pulpits, aflame with righteousness, did I understand the secret of her genius and power. America is great because she is good, and if America ever ceases to be good, America will cease to be great.”

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