Following Boston's Great Fire of 1872, the firm was enlisted in the reconstruction of many downtown buildings. A significant part of Cummings and Sears practice focused on ecclesiastical architecture, building churches throughout Massachusetts and northern New England. In 1874 Cummings was commissioned to design what is considered his masterwork, a new building for the third oldest congregation in America, Old South Church in Boston located in Boston's Copley Square. Also completed in the same year was the Bedford Block which is an example of Venetian Gothic architecture.