One can only marvel at Mr. Runciman's scholarship, his knowledge of Eastern history and languages was remarkable; he was acquainted with Latin, Greek, Russian, French, Arabic and many other Balkan languages. The sources he consulted and quoted are exhaustive and sometimes not easily accessible. He can only be commended for the depth of his research and making known to the reader the existence of a vast literature dealing with the subject covered in this book.
Nevertheless, it must be remembered that Mr. Runciman was a secular historian dealing with a religious subject, this obviously leads him to some unjustified conclusions. Despite his obvious erudition, the author fails to give us an adequate assessment of those he ranked among the Medieval Manichee. His appreciation of these people is often pejorative and even derogatory at times.
The sources available and used by Mr. Runciman are almost all hostile, since the opponents and in particular the Inquisition made sure that all the writings of those called Manichee were destroyed. All historians know this fact and should therefore be very careful in quoting them. The information gathered by those who dealt directly with these people and which was included in their histories, reports, letters and other writings was bias and therefore largely unreliable.
When one desires to write objectively about a movement like say the Paulicians, he should be aware of these well known facts and take them into account so as to present an accurate picture of his subject. It is clear that in his book Mr. Runciman took many sources he read for granted. He failed to weigh things in the balance of objectivity and sometimes he even contradicts common sense. He repeats many of the aberrations that were written a foretime without intimating that his sources could be questionable.
Historically this book is helpful for its broad outline and bibliographical notes. But it is my personal view that it fails totally to tell us the true story of these "Manichee".