The outbreak of way in 1562 was the culmination of a decade of extraordinary growth in French Protestantism. There may have been two million adherents in around a thousand congregations by 1562, while in the early 1550s there had been only a handful of secret groups; the phenomenon is even more spectacular in scale than the sudden emergence of popular Protestantism in Scotland in the same years that had so astonished John Knox. How had such rapid expansion taken place? Public preaching had not been possible on a significant scale to spread the message in France; there had not been enough ministers, and limited opportunities to gather to listen to sermons. Books played a major part, but the two central texts, the Bible and Calvin’s Institutes, were bulky and expensive and could not have had a major circulation in the years of persecution before 1560, while a massive increase in Bible publication came only after 1562. Lesser, more easily concealed pamphlets could be more easily distributed and read, but in one respect the Protestant crowds who emerged to fight their Catholic neighbors ignored what Calvin and the ministers of Geneva wrote……The explanation for this mass lay activism may lie in the one text which the Reformed found perfectly conveyed their message across all barriers of social status and literacy. This was the Psalter, the book of the 150 Psalms, translated into French verse, set to music and published in unobtrusive pocket-size editions which invariably included the musical notation for the tunes…….they were redeployed in Reformed Protestantism in this metrical form to articulate the hope, fear, joy and fury of the new movement. They became the secret weapon of the Reformation not merely in France but wherever the Reformed brought new vitality to the Protestant cause.
– Diarmaid MacCulloch, The Reformation: A History (Penguin Group), 307.