Between verses 15 and 16 [in Matthew 3] two Latin manuscripts . . . describe the baptism of Jesus as follows . . . (“And when Jesus was being baptized a great light flashed (a tremendous light flashed around) from the water, so that all who had gathered there were afraid”). According to Isho’dad of Merv (ninth century) and Dionysius Barsalibi (twelfth century), Tatian’s Diatessaron also contained a reference to the light. The passage from Isho’dad’s Commentary on the Gospels is as follows:

“And straightway, as the Diatessaron testifies, a great light shown, and the Jordan was surrounded by white clouds, and many troops of spiritual beings were seen singing praises in the air; and the Jordan stood still quietly from its course, its waters not being troubled, and a scent of perfumes wafted from thence; for the Heavens were opened” (M. D. Gibson’s translation, p. 27).

How much of this extract should be regarded as Tatianic, and how much may have been taken from other sources (perhaps an early hymn), is not known, but it is thought that, in view of Ephraem’s remark about “the shining of the light upon the waters” (Com. iv.5), at least the reference to the light on the Jordan was present in the Diatessaron.

Several other writers refer to the tradition of the light, including Justin Martyr, who says that after Jesus had gone down into the water “a fire kindled in the Jordan” (. . . Dial. c. Tryph. 88), and Epiphanius, after the voice came from heaven, “immediately a great light shone around the place” (. . . Panarion haer. xxx, xiii, 7).

Bruce M. Metzger, A Textual Commentary on the Greek New Testament, 10.