MLA
Grain of Mustard Seed: A Letter to the Rev. Mr. John Wesley at Founding the Benevolent or Strangers' Friend Society. 1785. Pitts Theology Library, Box 40, Folder I95-001-207. https://digital.library.emory.edu/purl/0171vhhn0j-cor.
APA, 6th edition
Grain of Mustard Seed: A Letter to the Rev. Mr. John Wesley at Founding the Benevolent or Strangers' Friend Society (Box 40, Folder I95-001-207). (1785). [Correspondence]. Pitts Theology Library. https://digital.library.emory.edu/purl/0171vhhn0j-cor.
Chicago
“Grain of Mustard Seed: A Letter to the Rev. Mr. John Wesley at Founding the Benevolent or Strangers' Friend Society.” Correspondence. Wesleyana Collection, 1785. Box 40, Folder I95-001-207. Pitts Theology Library. https://digital.library.emory.edu/purl/0171vhhn0j-cor.