Albert Barnes Commentary Matthew 1:25

"and knew her not till she had brought forth a son: and he called his name JESUS." — Matthew 1:25 (ASV)

Knew her not. The doctrine of the virginity of Mary before the birth of Jesus is a doctrine of the Scriptures, and very important to be believed. But the Scriptures do not affirm that she had no children afterwards. Indeed, all the accounts in the New Testament lead us to suppose that she had. See Barnes on Matthew 13:55-56.

The language here evidently implies that she lived as the wife of Joseph after the birth of Jesus.

Her firstborn son. Her eldest son, or he who by the law had the privilege of birthright. This does not of necessity imply that she had other children, though it seems probable. It was the name given to the son who was first born, whether there were others or not.

His name JESUS. This was given by Divine appointment, Matthew 1:21. It was conferred on him on the eighth day, at the time of his circumcision, Luke 2:21.