For dialect enthusiasts and language buffs, Google’s NGram Viewer is the kind of thing that keeps us up till the wee hours. For those out of the loop: type any word (or phrase) into Google’s NGram search, and you’ll get a graph of how frequently this word was used in literature for a give time frame. Cool!
Some fascinating things can be gleaned here. First, thou seems to have had a ‘final hurrah’ during the golden age of Elizabethan literature. Most of what I’ve read suggests that the word was dying out in London by the time of Shakespeare. Paradoxically, this same time seems to be the golden era of literary ‘thou.’ So while the word may have been uncommon on the streets, writers of the period preserved it.
I was surprised to see such a precipitous drop in the early 1600’s given the publication of the King James Bible. But notice the spike in thou usage in the mid-17th-Century. Looking at a more close-up view of the chart, this increase almost perfectly aligns with the publication of the revised Book of Common Prayer in 1662. (For Americans, this is the prayer book used by the Anglican church.) So it seems this book, rather than the King James Bible itself, led to the notion that God uses the second person singular.
This publication seems to lead to a brief increase in liturgical thou: Browsing over the Google Book results from 1650-1700, the vast majority of thou appearances are in various religious texts. Then, sharply at the turn of the 18th-Century, thou falls out of usage. From 1700 till the present, the word is presumably reserved for references to the Bible, Elizabethan drama, or various English dialects. Thou shalt never again relive thy heyday in the time of Marlowe and Shakespeare.