As a Christian apologist of the mid-Nineteenth century, Greenleaf was one of many writers who contributed to the debates that ensued on both sides of the Atlantic concerning the historicity of the gospel accounts in general, and specifically the miracle of the resurrection of Jesus Christ. Part of his argument relied on earlier Christian apologists such as William Paley, Thomas Hartwell Horne, and Mark Hopkins, and he cites their works in "The Testimony of the Evangelists". Here he followed the basic appeals to logic, reason, and historical evidences on behalf of the Bible generally, and in defence of the possibility of miracles occurring.