What would the reason be for adding ‘she brought forth her firstborn son?’ [in Matthew 1:25]. It is claimed that it was that the redactor wanted to buttress the teaching of the virgin birth. Now this is obviously a possibility, if only the textual evidence for Matthew supported it. But what effectively blows the argument clean out of the water is that the same phrase is found in Lk. 2:7, and in that case there is absolutely not a shred of textual evidence for saying that it has been added by a copyist. No Lucan text misses the statement out. Because of this, the very translators who opt to leave it out of Matt. 1:25 are stuck with it in Lk. 2:7.
If the text is left untampered with, what is Matthew seeking to say through this 'unnecessary' statement? Being a very Jewish story for the Jewish church, it is clearly marking Jesus as the Lord's firstborn. Every Jewish listener of the story of Matthew would instinctively know that this is the Lord's redeemer, the one who will save his people from their sins. Jesus is being introduced as the child born to die, the king, the firstborn, whose destiny is suffering to bring salvation to his people.
This is certainly supported by the way Luke uses the same phrase. He also, using the logic of the modern translators, unnecessarily repeats himself, or does he? There are a number of strange things in the account of Luke that are not so strange once we have the reason for this statement sorted out. Why do Joseph and Mary take the baby Jesus to the temple? It was next to the palace of Herod, the very seat of his government, and they knew that he sought the life of the child. It was not that they had to do it fulfil the law. Simply paying the prescribed half shekel to a scribe could redeem the firstborn child. In fact, despite saying that everything was done according to the requirement of the law, it clearly referred to the purification of Mary, for that is what the sacrifice of the doves was to achieve. What is not mentioned, and this is incredible considering it would be the most important thing that every Jewish couple were to do on the birth of their firstborn, is that they never redeemed the child. The child was no longer Joseph and Mary's, but he was the Lords firstborn. This makes sense of Mary singing Hannah's song for she also gave her son to the Lord. It also explains why Jesus was surprised that Joseph and Mary had not expected him to be in the temple when he was found to be missing from the returning pilgrim party. It also explains why he should say that Mary was not his mother. The natural ties had been severed because they had not redeemed him. This of course suggests that Jesus was conscious of a priestly calling from his youth.
Holland, Tom. Contours of Pauline Theology. London, UK: Christian Focus Publications, 2004. 270-71.