Jack Markwardt explains:
I originated and presented this hypothesis to an international conference convened at Ohio State University in 2008 for the simple reason that the early history of the Turin Shroud cannot be credibly linked to the ancient city of Edessa through a literal application of the Abgar legend. The preeminent historian of Edessa, J.B. Segal, after years of arduous study and investigation, concluded that the Abgar legend constitutes “one of the most successful pious frauds of antiquity”. It should not be surprising, therefore, that a number of highly-respected modern historians have summarily rejected this pious fraud as evidential of the Turin Shroud’s whereabouts during the first Christian millennium, particularly because real historical evidence provides not the slightest indication that pagan Edessa was even partially converted to Christianity prior to the late second-century reign of King Abgar the Great. The preeminent historian of Antioch, Glanville Downey, ascribed that development to a two-phase evangelization mission, one which initially resulted in the baptism of Abgar the Great and ultimately concluded with the consecration of Edessa’s first bishop, Palut, in 200 CE, by Serapion, the bishop of Antioch. Relatively recent attribution of an image of Christ to the city of Edessa during the first half-millennium of Christianity arises exclusively from a substantial permutation of the Abgar legend authored, in ca. 945, by a Byzantine Emperor who desired to bestow an apostolic provenance upon the Christ-icon which had recently been transferred to his capital from Edessa. In order to explain away, in one bold imperial stroke, the complete historical anonymity of this icon during the first five Christian centuries, Constantine VII Porphyrogenitus sponsored the publication and circulation of a tale which featured not only the cloth’s first-century concealment within a niche located above an Edessa city gate, but also its miraculous rediscovery there by a fictional Edessan bishop, Eulalius, during the Persian siege of 544 CE. It has been suggested, in lieu of this incredible miracle-discovery tale, that the icon was actually found in the wake of the great flood of 525 CE which damaged Edessa’s city walls; however, it is rather significant that such a truly notable event merited no mention whatsoever in the Edessan Chronicle, a Syriac work composed in ca. 540-544 CE, which not only described the great flood but also detailed the most commonplace of Edessan ecclesiastical matters. In my opinion, modern scholarship will continue to reject the identification of the acheiropoietos image of Christ which was brought from Edessa to Constantinople in 944 CE with the Turin Shroud unless and until the provenance of that icon, and the circumstances surrounding its arrival in Edessa, can be reasonably established on the basis of non-legendary evidence.
I was there at Ohio in 2008 and remember the presentation, Ancient Edessa and the Shroud: History Concealed by the Discipline of the Secret. It was excellent. Read it.