Runciman is still hugely popular and influential even today. For some people, his three-volume History of the Crusades may be the only book they ever read about the crusades. Runciman also wrote some of the early chapters of the 6-volume History of the Crusades published by the University of Wisconsin (1965-1985, the “Wisconsin history”) so his influence extended into the 80s at least (although his chapters are mostly just copied from his own history). Popular culture often gets its ideas about the crusades directly from Runciman; I know I’ve mentioned here on AH in the past that some of the overall themes of the 2005 movie Kingdom of Heaven, and even some of the individual scenes, come straight out of Runciman (the screenwriter even cites him the DVD commentary!).

I don’t have any specific numbers but I’d bet most English-Speaking crusade historians started off reading Runciman when they were younger. I still have my well-worn Folio Society edition.

Unfortunately, it’s not just a matter of specific ideas that are wrong. He was just a very old-fashioned writer. Even he himself admitted that he felt more like a storyteller than an historian. He relied on literary sources almost exclusively - some through translation into modern languages, but he had also mastered ancient and medieval languages so he was able to read more obscure stuff that other historians couldn’t. But medieval chronicles are just one source, which are very important, but they have their own biases and agendas, and historians today like to balance them out with other kinds of evidence: poetry, songs, non-narrative texts like charters, contracts, wills, and also archaeological findings. All of those things were available to Runciman but he mostly either didn’t know about them or simply ignored them, for the sake of telling a coherent story.

“The questions he asked about the past were answered primarily from printed narrative sources, and he was not particularly concerned with many of the questions that interest scholars today…As a Byzantinist, he concentrated on the expeditions to the East and did not take into account the crusades in Spain and northeastern Europe, nor those against Christians and heretics within Europe. Nor did he discuss the motives or ideas of the crusaders, the financing of the expeditions, or the social and institutional structure of the crusader states in the East. His primary concern was with military and political events and with the great men who made the history of the crusades into a mighty epic.” (Constable, pg. 100)

His love for Byzantium is a well-known bias:

“The author was, first and foremost, a historian of the Byzantine Empire and for him, the sack of his beloved city of Constantinople by the Fourth Crusade in 1204 was the ultimate act of betrayal by a bloodthirsty and misguided movement. The contrast with the sophistication of Byzantium and Islam could not have been plainer and the crusaders were seen as ignorant thugs. The violence of the crusading age cannot, and should not, be denied, yet this must be seen in the context of western European society of the time and through proper examination of contemporary motives and values.” (Phillips, pg. 4)

Sometimes historians are willing to admit that, even if it’s inaccurate and unsophisticated, his history is at least exciting to read, but not everyone agrees:

“Even when published, dated in technique, style and content; derivative, misleading and tendentious; a polemic masquerading as epic” (Tyerman, pg. 192)

Tyerman even feels that Runciman is not useful as a secondary source at all; his History is actually “the last chronicle of the crusades” (pg. 196), with all of same problems and biases that we have to untangle when reading a medieval author.

It's like Runciman was almost displaced in time, a rich English aristocrat accidentally transplanted to the 20th century. His famous conclusion that the crusades were “a sin against the Holy Ghost”, is an odd approach from a supposedly modern historian. Tyerman also notes Runciman's belief that “there never was a greater crime against humanity than the Fourth Crusade” - but for someone writing in the 1950s, surely this could not be true?! Had he missed the previous couple of decades?

So it’s not so much that specific ideas or statements are wrong, it’s just that his entire outlook on life and his philosophy of history and history-writing were totally out of date even in his own lifetime.

It would actually be pretty simple for an expert historian to turn to any random page and find something suspicious, although non-experts probably wouldn't notice.For example, there was a dispute recently (on Wikipedia, of course) about how Godfrey of Bouillon and his brother Baldwin I of Jerusalem were related to Baldwin II of Jerusalem - medieval sources just say they were related but don’t say how. But in Runciman you’ll find that Baldwin II’s grandmother was Godfrey/Baldwin I’s aunt, even though there’s no evidence for that at all. Did he make it up? Maybe…

Another example of this is his belief that “John Grant”, a mercenary at Constantinople in 1453, was Scottish (something I wrote about here on AH). Not coincidentally, Runciman had Scottish ancestry and lived in a castle in Scotland…

And to add one last personal anecdote, I was taking a Byzantine history class in university when Runciman died. The professor had been one of his students, so he spent a whole lecture telling us stories about him. The story I remember best is that Runciman once heard someone speaking Thai, thought it was a beautiful language, and so he learned to speak it as well. He was an old-fashioned eccentric with all the time and money in the world to do whatever he wanted!

Sources:

David Abulafia, “Sir Steven Runciman (1903-2000): A Memoir”, in Mediterranean Studies, Vol. 9 (2000), pp. 1-16

Giles Constable, “Sir Steven Runciman, 7 July 1903 - 1 November 2000”, in Proceedings of the American Philosophical Society, Vol. 147, No. 1 (Mar., 2003), pp. 95-101

Jonathan Harris, The End of Byzantium (Yale University Press, 2012)

Jonathan Phillips, The Crusades: 1095-1197 (Longman, 2002)

Jean Richard, “National feeling and the legacy of the crusades”, in Palgrave Advances in the Crusades, ed. Helen J. Nicholson (Palgrave Macmillan, 2005)

Christopher Tyerman, The Debate on the Crusades (Manchester University Press, 2011), particularly chapter 7, “Erdmann, Runciman and the end of tradition”