Justin occasionally uses the Gospel of Matthew directly as a source for Old Testament prophecies to supplement his testimony sources. However, the fulfillment quotations from these sources most often appear to be harmonizations of the gospels of Matthew and Luke. Koester suggests that Justin had composed an early harmony along the lines of his pupil Tatian's Diatesseron. However, the existence of a harmony independent of a collection of sayings for exposition purposes has been disputed by scholar Arthur Bellinzoni. The question of whether the harmonized gospel materials found in Justin's writings came from a preexisting gospel harmony or were assembled as part of an integral process of creating scriptural proof-texts is an ongoing subject of scholarly investigation. The "kerygma source"
The following excerpt from 1 Apol. 33:1,4-5 (partial parallel in Dial. 84) on the annunciation and virgin birth of Jesus shows how Justin used harmonized gospel verses from Matthew and Luke to provide a scriptural proof of the messiah-ship of Jesus based on fulfillment of the prophecy of Isaiah 7:14. "And hear again how Isaiah in express words foretold that He should be born of a virgin; for he spoke thus: 'Behold, the virgin will conceive in the womb and bear a son, and they will say in his name, God with us' (Mt 1:23)." (1 Apol. 33:1) "...the power of God, coming down upon the virgin, overshadowed her and made her while yet a virgin to conceive (cf. Lk 1:35), and the angel of God proclaimed to her and said, 'Behold, you will conceive in the womb from the Holy Spirit and bear a son (Mt 1:20/Lk 1:31) and he will be called Son of the Most High (Lk 1:32). And you shall call his name Jesus, because he will save his people from their sins (Mt 1:21),' as those who have made memoirs of all things about our savior Jesus Christ taught... (1 Apol. 33:4-5)