Is the letter authentic?
For almost two millennia, Eusebius' 4th century Ecclesiastical History was the principal witness for the "Edessene legend". Eusebius devoted an entire chapter of his book to a "Narrative concerning the Prince of the Edessenes". In it he relates how "Abgarus", king of the city of Edessa in Asia Minor (modern day Şanlıurfa, in Turkey, today, near the Syrian border), "afflicted with a terrible disease which it was beyond the power of human skill to cure, when he heard of the name of Jesus, and of his miracles, which were attested by all with one accord sent a message to him by a courier and begged him to heal his disease."
According to Eusebius, the text of Abgar's letter read:
Abgarus, ruler of Edessa, to Jesus the excellent Saviour who has appeared in the country of Jerusalem, greeting. I have heard the reports of thee and of thy cures as performed by thee without medicines or herbs. For it is said that thou makest the blind to see and the lame to walk, that thou cleansest lepers and castest out impure spirits and demons, and that thou healest those afflicted with lingering disease, and raisest the dead.
And having heard all these things concerning thee, I have concluded that one of two things must be true: either thou art God, and having come down from heaven thou doest these things, or else thou, who doest these things, art the Son of God.
I have therefore written to thee to ask thee that thou wouldest take the trouble to come to me and heal the disease which I have. For I have heard that the Jews are murmuring against thee and are plotting to injure thee. But I have a very small yet noble city which is great enough for us both.
In reply, Jesus supposedly wrote:
Blessed art thou who hast believed in me without having seen me.1 For it is written concerning me, that they who have seen me will not believe in me, and that they who have not seen me will believe and be saved.2 But in regard to what thou hast written me, that I should come to thee, it is necessary for me to fulfill all things here for which I have been sent, and after I have fulfilled them thus to be taken up again to him that sent me. But after I have been taken up I will send to thee one of my disciples, that he may heal thy disease and give life to thee and thine.”
Eusebius explains that his source for this account was the "archives of Edessa":
You have written evidence of these things taken from the archives of Edessa, which was at that time a royal city. For in the public registers there, which contain accounts of ancient times and the acts of Abgarus, these things have been found preserved down to the present time. But there is no better way than to hear the epistles themselves which we have taken from the archives and have literally translated from the Syriac language.
He also states that in addition to the supposed letter and reply, "was added the following account in the Syriac language", which begins:
“After the ascension of Jesus, Judas, who was also called Thomas, sent to him Thaddeus, an apostle, one of the Seventy3. When he was come he lodged with Tobias,the son of Tobias. When the report of him got abroad, it was told Abgarus that an apostle of Jesus was come, as he had written him.
Thaddeus began then in the power of God to heal every disease and infirmity, insomuch that all wondered. And when Abgarus heard of the great and wonderful things which he did and of the cures which he performed, he began to suspect that he was the one of whom Jesus had written him, saying, ‘After I have been taken up I will send to thee one of my disciples who will heal thee.’
Some have suggested that the entire Edessene legend was fabricated by Eusebius, but this is probably not the case. A footnote to the account in Schaff's Nicene and Post-Nicene Fathers volume from which the account above comes states:
We have no reason to doubt that Eusebius, who is the first to mention these apocryphal epistles, really found them in the public archives at Edessa. Moses Chorenensis, the celebrated Armenian historian of the fifth century, who studied a long time in Edessa, is an independent witness to their existence in the Edessene archives. Eusebius has been accused of forging this correspondence himself; but this unworthy suspicion has been refuted by the discovery and publication of the original Syriac (The Doct. of Addai the Apostle, with an English Translation and Notes, by G. Phillips, London, 1876; compare also Contemp. Rev., May, 1877, p. 1137). The epistles were forged probably long before his day, and were supposed by him to be genuine. His critical insight, but not his honesty, was at fault. The apocryphal character of these letters is no longer a matter of dispute, though Cave and Grabe defended their genuineness (so that Eusebius is in good company), and even in the present century Rinck (Ueber die Echtheit des Briefwechsels des Königs Abgars mit Jesu, Zeitschrift für Hist. Theol., 1843, II. p. 326) has had the hardihood to enter the lists in their defense; but we know of no one else who values his critical reputation so little as to venture upon the task.
As the note indicates, ancient copies of the account in Syriac were acquired in the mid-19th century by the British Museum. One editor of a publication featuring the documents, a Dr. W. Wright of the British Museum, wrote:
I have found among the Syriac MSS. in the British Museum a considerable portion of the original Aramaic document which Eusebius cites as preserved in the archives of Edessa, and various passages from it quoted by several authors, with other testimonies which seem to be sufficient to establish the fact of the early conversion of the inhabitants of that city, and among them of the king himself, although his successors afterwards relapsed into paganism. These, together with accounts of the martyrdom of some of the first bishops of that city, forming a most interesting accession to our knowledge of the early propagation of Christianity in the East down to about A.D. 300, I have already transcribed, and hope to publish4
The original editor, Dr. William Cureton, wrote Dr. Wright, "was himself firmly persuaded of the genuineness of the Epistles attributed to Abgar, king of Edessa, and our Lord: an opinion which he shared with such illustrious scholars as Baronius, Tillemont, Cave, R. Mountague (Bishop of Norwich), and Grabe."
I do not offer any of the above as firm evidence that the document is authentic. But it is an example of the seriousness with which some scholars take it (or took it at one time).
If the letter is authentic, why isn't it in the New Testament?
Some take the fact that the letter is not in the New Testament as evidence in and of itself that it is not authentic. But this is not necessarily the case. The end of John's Gospel concludes, But there are also many other things which Jesus did; were every one of them to be written, I suppose that the world itself could not contain the books that would be written. John does not say that these things never did come to be written.
If we look at the text of the letter, it is a very short personal message to Abgar. Unlike the Epistles of the Apostles, it does not really touch on somewhat broad themes applicable to the entire Church. I think this may be the main reason why, if authentic, it was not included.
This is not to say, however, that even if it is not suitable for inclusion in the canonical New Testament, that, if authentic, it is not somehow inspired and worthy of attention. Syrian Orthodox liturgies commemorate the correspondence of Abgar during Lent and parts of the letter are quoted in the Old Irish Liber Hymnorum. The legend of Abgar is also included in all the Orthodox hagiographies of Thaddeus as well.
1. cf John 20:29
2. cf Isaiah 6:9, Jeremiah 5:21, Ezekiel 12:2
3. Luke 10:1-24
4. Introduction to Memoirs of Edessa and Other Syriac Documents