Sure. It's from a book titled The Twelve. When I return home tonight, I'll post the part of the book dealing with this particular letter.
The following is from C. Bernard Ruffin's The Twelve
. I stole the Wikipedia translation of Agbar's letter because I didn't feel like typing the whole thing out. Everything else are the words of Ruffin. As for why this isn't in the Bible, remember S. Jo. writes about how Christ did so many great things that there wouldn't be enough books in the whole world to record it all. Chances are that this letter was left on the cutting room floor, so to speak. Here goes,
"Thomas had one of the most active ministries of any of the Twelve. Eusebius wrote that almost immediately after Pentecost, Thomas was instrumental in evangelizing the nation of Osroene, which lay to the north of Palestine, in what is now eastern Turkey, between the Roman and Iranian Empires. Eusebius cited documents which still existed in his day in the Archive of Edessa, the capitol of Osroene, which described the conversion the conversion of Prince Abgar the Black, who ruled there between A.D. 13 - 50. Among those papers was a transcript of a letter which Abgar had written to Jesus. Abgar was ill with a distemper, impervious to the art of the physicians of his day, and wrote to Jesus that he had heard 'about the cures you perform without drugs and herbs.' He continued,
'I have heard of Thee, and of Thy healing; that Thou dost not use medicines or roots, but by Thy word openest (the eyes) of the blind, makest the lame to walk, cleansest the lepers, makest the deaf to hear; how by Thy word (also) Thou healest (sick) spirits and those who are tormented with lunatic demons, and how, again, Thou raisest the dead to life. And, learning the wonders that Thou doest, it was borne in upon me that (of two things, one): either Thou hast come down from heaven, or else Thou art the Son of God, who bringest all these things to pass. Wherefore I write to Thee, and pray that thou wilt come to me, who adore Thee, and heal all the ill that I suffer, according to the faith I have in Thee. I also learn that the Jews murmur against Thee, and persecute Thee, that they seek to crucify Thee, and to destroy Thee. I possess but one small city, but it is beautiful, and large enough for us two to live in peace.'
According to the papers on file in the Eddesan Archive, Jesus sent Abgar the following reply, orally, through the messenger who had dispatched the letter from the prince: "Happy are you who believed in me without having seen me! For it is written of me that those who have seen me will not believe in me, and that those who have not seen will believe and live. As to your request that I should come to you, I must complete all that I was sent to do here, and, on completing it, must at once be taken up to the One who sent me. When I have been taken up I will send you one of my disciples to cure your disorder and to bring life to you and to those with you."
Ruffin, C. Bernard. 1984. The Twelve: The Lives of the Apostles After Calvary
. Huntingto, Indiana: Our Sunday Visitor, Inc. 124-5