Final answer:
The aspect of the birth of Jesus in Christian mythology that matches the monomythic hero model is the announcement and identification of Jesus as the Messiah in the Jewish temple.
Step-by-step explanation:
The aspect of the birth of Jesus in Christian mythology that most clearly matches the monomythic hero model prior to the hero beginning their heroic journey is the announcement and identification of Jesus as the Messiah in the Jewish temple. In this event, Mary and Joseph bring the newborn Jesus into the temple, where the prophets Simeon and Anna recognize him as the Messiah, marking the beginning of his divine mission and the fulfillment of ancient prophecies.