National Register of Historic Places on May 1, 1974, citing the historic significance of the architecture of the area and the importance of the Congregational House and Library as a place "...to protect the original Puritan literature." Bas-Reliefs During design and construction the Building Committee of the American Congregational Association agreed on a series of four bas-reliefs should be included in the second story facade of the Congregational house, with each relief depicting a fundamental principle of Congregationalism. The tablets are carved from Knoxville marble and bear no inscription as it was assumed the subject of each panel would be known on sight to the average citizen. The reliefs were carved by Domingo Mora, a Spanish sculptor who, catalyzed by the Spanish–American War, left the country and reliefs unfinished, citing his inability to stay and work in a country hostile to his own. The reliefs were carefully completed according to Mora's design by a Swiss modeler known only as Mr. Stadler. Collections and Programs Holdings While the Congregational Library & Archives was founded as a library for Congregational ministers, it has evolved into a research library and repository for some 250,000 books, pamphlets, and periodicals, documenting the growth and development of the Congregational tradition in the United States, intimately bound up with early American history and the social movements in which Congregationalists actively participated such as abolition, temperance, and women's suffrage. As the designated archive of the Congregational Christian churches, the Library collects material on the history of the denomination from the Puritans to its present incarnation, holding all significant institutional records from the United Church of Christ, the National Association of Congregational Christian Churches, and the Conservative Congregational Christian Conference, before and after. The Library also holds some 1,500 different periodicals representing its longstanding interest in social reform, missionary work, and education. The Congregational Library also holds rare newspapers from the Christian Connection denomination. The Library's rare book room includes an unusually rich and complete representation of English and first-generation Puritan works, including an original copy of the Cambridge Platform of 1649. The Library's archival collection of colonial-era church records is also extensive, containing many sets of seventeenth-century documents as well as full collections from large and historically significant modern churches like Boston's Old South Church and Park Street Church. Many of these records have been digitized and made accessible as part of the New England's Hidden Histories program.