Heidegger's Bible Handbook: NT Apocrypha: Letter of Christ to Abgar, Part 2
6. And it is recalled to the anvil.
Baronius, on 31 AD, Section 58, and of our men ἐπαμφοτερίζων/ double-tongued Montagu, Originibus Ecclesiasticis, Tome II, part 2, commend them; yet not daring to place them among the Holy Writings. Neither did others, including Bellarmine, de Scriptoribus Ecclesiasticis, on the year 34 AD, and Costerus, Enchiridio, chapter 1, dare it, with a stupidity certainly marvelous. For indeed, if the Epistle was written by Jesus, who is so insane that he would dare to take from it Divine and Canonical authority, and to pronounce the things written by the Apostles Canonical, but the things written by Jesus Himself ἀκανόνιστα/non-Canonical? Especially if it obtains that dignity and force in the city of Edessa, which the Legend of the Apostle Thomas, and Petrus de Natalibus, Catalogo Sanctorum, book 4, chapter 43, attribute to it, so that from the time of its sending to Abgar no heretic, whether Jew or idolater, might be able to live there; and, with the reading of this Epistle heard throughout the city, would not enemies venture to hold that city as unsafe? But Gelasius far more wisely pronounced those Epistles Apocryphal. Upon Eusebius, certainly liable in other matters to excessive credulity, either some Prefect of the Archives of Edessa wrought some deceit, as the Most Learn Vossius conjectures, Harmoniæ Evangelicæ, book II, section 6; or a precipitous judgment concerning the γνησιότητι/genuineness of these Epistles imposed. For where are those words found written, that those seeing are not going to believe, and those not seeing are going to believe? Even indeed in John 20:29, Christ after His resurrection said to Thomas: Blessed are they that have not seen, and yet have believed. But this Epistle, if indeed it is genuine, preceded by some years that reproof of Thomas, especially as committed to writing. Would the Evangelists have passed over in silence so illustrious a legation of the King to Christ? for the Evangelists described in detail far lesser matters, and the writing of such a thing was more fitting for them than for Eusebius many years after. Moreover, that in the time of Christ there was a King of Edessa, is not able to be proven. Nor, if there was such a King, is it able to be said that Christ, who did not send the Canaanite woman away from himself without help, indeed promised in John 6:37, that He is not going to cast out the one coming to Him, was not going to esteem the prayers of this King as approved. I pass over in silence, both the variety of the edition of Greek and Arabic, which even he that passes by will find: and the γραώδεις μύθους, old-wives’ tales, or the associated empty fables, and among those especially that of the image of Christ sent by Himself together with the Epistle to Abgar; that is, in order to gratify this one that Epistle also so sweetly smiles upon icon-worshippers.
 Richard Montagu (1577-1641) was an Anglican controversialist and bishop.  Petrus de Natalibus (died c. 1400) was an Italian bishop and hagriographer.