Another prominent man among the heretical writers was Tatian, a contemporary and pupil of Justin Martyr, who, according to the testimony of Eusebius, Epiphanius, and Theodoret, composed a Diatessaron, that is, a four-fold gospel; which can be understood only as a harmony of the four gospels which, as has been shown, were used by Justin; or of such parts of these gospels as suited his purpose; for Tatian, like Marcion, omitted all that relates to our Lord's human descent.
"Companion to the Bible"
E. P. Barrows
With this Diatessaron, Theodoret was well acquainted; for he found among his churches more than two hundred copies, which he caused to be removed, and their places supplied by the four canonical gospels.
"Companion to the Bible"
E. P. Barrows