It discusses the "star" prophecy of Num 24:17 and how it originated and was passed on down to the Magi of Matt 2. It seems to me in a way like "fan service", a back story for people who couldn't get enough of Matt 2. After describing how the prophecy was preserved and handed down by "the Persians", finally reaching the Magi, it segues into the gospel story. It follows the account in Matthew 2 but differs in some essentials (no meeting with Herod, no angel warning Joseph to flee, "Egypt" not specifically named, no slaying of infants, no mention of Archelaus, when the family goes to Galilee they have "the five sons of Hannah, the first wife of Joseph" in tow). Then it attempts to tie this narrative in with the Annunciation to Mary. The writer is apparently using the Peshitta version of the OT and NT, per Benjamin Bacon.
Then, sixteen lines of the Syriac are erased. Per the translator, probably because of a statement later deemed heretical.
Then we get this:
"[lacuna] and eleven, in the second year of the coming of our savior, in the consulship of Caesar and of Capito, in the month of the later Kanun, these Magi came from the east and worshiped our Lord at Bethlehem of the kings. And in the year four hundred and thirty (119 AD), in the reign of Hadrianus Caesar, in the consulship of Severus and of Fulgus, in the episcopate of Xystus, bishop of the city of Rome, this concern arose in [the minds of] men acquainted with the Holy Books; and through the pains of the great men in various places this history was sought for and found, and written in the tongue of those who took this care. Here ends the Discourse on the Star, which was composed by Mar Eusebius of Caesarea."
It was difficult for me to imagine how they could have specifically come up with the 430 (year of Seleukos Nikator) / 119 AD date for this meeting of the minds where church leaders essentially investigated Matthew 2 and its backstory. It would appear that Xystus was plugged in from Christian chronologies. Per Benjamin Bacon, "Severus and Fulgus" come from the fifth century Consularia Constantinopolitana, where they are listed as the consuls for 120 AD. In a 1929 article, "As to the Canonization of Matthew," Bacon postulated that the writer lifted these synchronisms from some report of a synod in 120 AD that may have had some bearing on the canonization of that gospel.
But my thinking is this:
The writer(s) of the Discourse had access to some copy of Matthew with which the 119 AD date was associated, whether a note or a gloss. This might make sense, given the supposed Syrian provenance of Matthew. They didn't know what to make of it, and their thought process may have been:
1. We really like this "star" story
2. It bothers us that only Matthew mentions it
3. We want to write a defense of it
4. We also have this puzzling 430 / 119 date associated with Matthew; it can't be the date the gospel was written because Matthew was an eyewitness
5. 430 / 119 must have been the date when the Roman church officially examined Matthew's gospel and said that his star story was true and correct
6. We'll go ahead and make use of that date in our "Discourse"
2. It bothers us that only Matthew mentions it
3. We want to write a defense of it
4. We also have this puzzling 430 / 119 date associated with Matthew; it can't be the date the gospel was written because Matthew was an eyewitness
5. 430 / 119 must have been the date when the Roman church officially examined Matthew's gospel and said that his star story was true and correct
6. We'll go ahead and make use of that date in our "Discourse"
One would think that if they wanted solely to establish the canonicity of the star story, the episcopacy of Xystus seems an odd and even a random choice. If they were following the legends that the gospels were written after the death of Peter, they could have still reverted to Linus, Clement, et al. Especially since they were using the same succession lists that feature Xystus. Alternatively, the 430 / 119 may have been associated with Matt 1-2 only if that is a later addition.