*[http://books.google.com/books?id=WMIMAAAAIAAJ&printsec=toc&dq=intitle:%22social+gospel%22&num=30&as_brr=1&sig=aLOdXOBESYpf7sdpP3jQspONW9 Essays on the social gospel]
+
*[https://books.google.com/books?id=WMIMAAAAIAAJ&printsec=toc&dq=intitle:%22social+gospel%22&num=30&as_brr=1&sig=aLOdXOBESYpf7sdpP3jQspONW9 Essays on the social gospel]
*[http://www.ccel.org/ccel/harnack/christianity.iii.ii.html What is Christianity?]
*[http://www.ccel.org/ccel/harnack/christianity.iii.ii.html What is Christianity?]
*[http://www.ccel.org/h/harnack/ Biography of Adolf Harnack.]
*[http://www.ccel.org/h/harnack/ Biography of Adolf Harnack.]