 This source uses the Gospel of Matthew only and no other known Gospel, and thus it must have been written before the four-Gospel canon had become widespread in the churches, i.e.before the second half of the 2nd century when Tatian produced the Diatessaron.Given its literary dependence on the Gospel of Matthew, it is not surprising that the Didache follows the Gospel of Matthew in designating the a triadic formula as the baptismal formula