The correspondence covers all aspects of Hillis's professional life. The bulk of the letters fall between 1913 and 1918 and relate to Plymouth Church business, especially plans for the Beecher Memorial and ongoing construction of the Plymouth Institute, and Hillis's personal and business affairs, lecture tours, and publications. A major portion of the correspondence is from parishoners and other followers of Hillis. These are generally letters of support and thanks, or requests for copies of sermons and publications. Hillis's replies to these letters are mostly absent from the series.