"The one who sings with me is my brother"

Here's Mark Noll on the unifying and divisive history of Christian hymnody:

An old German proverb runs: "Wer spricht mit mir ist mein Mitmensch; wer singt mit mir ist mein Bruder" (the one who speaks with me is my fellow human; the one who sings with me is my brother). In the world Christian community today, nothing defines "brotherhood" more obviously than singing. As it was in the beginning of the limited Christian pluralism in 16th-century Europe, so it remains in the nearly unbounded Christian pluralism of the 21st century. As soon as there were Protestants to be differentiated from Catholics, Calvinists from Lutherans, Anabaptists from Lutherans and Calvinists, Anglicans from Roman Catholics and other Protestants—so soon did singing become the powerful two-sided reality that it continues to be.

One reality was that believers who together sang the same hymns in the same way came to experience very strong ties with each other and even stronger rooting in Christianity. Psalm singing nerved Huguenots to face death and devastation during France's violent religious wars of the 16th and 17th centuries. Palestrina gave an incalculable boost to the Counter-Reformation when he provided masses, hymns, litanies, and magnificats that for many Roman Catholics became as expressive of their faith as the congregational singing of Protestants was of theirs. In his fine book Singing the Gospel, Christopher Boyd Brown has shown that Bohemian Lutherans survived several generations of imperial Catholic pressure because families and lay groups were so much strengthened by the hymnody of Luther and his tradition.

Anabaptism was a movement of song—non-instrumental, non-clerical, non-élitist—as well as movement of belief; when Brethren or Mennonites sang, unaccompanied and in free form, the hymns of Michael Sattler, who was martyred in 1527 for his Anabaptist beliefs, they were affirming who they were as Christian believers and who they were not.

Which brings us to the second reality. As much as hymn singing has always been one of the most effective builders of Christian community, it has also always been one of the strongest dividers of Christian communities. In the early decades of the Reformation, Calvinists broke with Lutherans over several important matters, but one was existentially apparent at every gathering for worship: the singing. Lutherans sang hymns that with considerable freedom expressed their understanding of the gospel (like Luther's "A Mighty Fortress" or "From Heaven High I Come to You"), and they often sang them with choirs, organs, and full instrumentation. Calvinists, by contrast, sang the psalms paraphrased and with minimal or no instrumental accompaniment (like the 100th Psalm, "All people that on earth do dwell," which was prepared by William Kethe for English and Scottish exiles who had taken refuge in Calvin's Geneva during the persecutions of England's Catholic Mary Tudor). However natural it may now seem for Protestant hymnals to contain both Luther's "A Mighty Fortress" and Kethe's "Old One Hundredth," in fact it took more than two centuries of contentious Protestant history to overcome the visceral antagonism to "non-scriptural" hymns that prevailed widely in the English-speaking world. It was even longer before organs, choirs, and instrumental accompaniment were accepted.