The record-retrieval effort is painstaking. Every summer, Dr. Cooper (no relation to the 19th-century author; his grandparents were just admirers) and Dr. Bendroth travel from church to church, trying to persuade ministers and lay leaders to part with treasured documents. In some cases, churches are excited to do so. But for some of the churches, letting go of documents is difficult — the papers, even if brittle and faded, are a form of patrimony, like silver and pewter communion vessels, to be treasured.

One evening this summer, Dr. Cooper visited the Federated Church of Sturbridge and Fiskdale, a congregation that resulted from a merger of several different churches and that is now affiliated with the United Church of Christ, the American Baptist Churches, and the Unitarian Universalist Association. The congregation’s records — at least those representing its congregational past, which began in 1736 — had been retrieved from a safe at a member’s office.

As about a dozen members of a church council sat on folding chairs and tired couches, Dr. Cooper made his pitch. He showed them an iPad photo of a recent church fire in Somers, Conn., as a cautionary tale, and he urged them to make a permanent, but revocable, loan of the records to the Congregational Library, which could store them in a climate-controlled rare book room. After a number of questions about security, access and backup plans, and a brief closed-door debate, the congregation agreed. Ms. Leighninger called the decision a relief and a godsend, because the records would be protected and available to genealogists, historians and anyone else through the library’s digital archive.

Dr. Cooper methodically went through dozens of documents, looking for those published before 1860, and choosing about 25 he deemed of historical significance. There were letters bound with string, accounts of “meetings of the brethren,” lists of “confessions for scandals and offenses,” and ledgers detailing the activities of the Sturbridge Ladies Benevolent Society. He packed them in boxes and loaded them in his red Prius before walking across the street to take pictures of the historic cemetery in the fading light.