Matthew 1:25

And knew her not till she had brought forth her first-born son: and he called His name Jesus.

Joseph did not enter into the natural relations of marriage with Mary until her son, the promised Messiah, had been born. It is a moot question whether Mary and Joseph ever lived together in the usual matrimonial intercourse and begot children. The Roman Catholic theologians and a great many Protestant commentators argue with much spirit that the first-born son of Mary was her only son. Some have held with one of the early Church Fathers that the "brethren" of Jesus mentioned in various passages. Matthew 12:46; Matthew 13:55; Mark 3:31; Luke 8:19; John 2:12; John 7:5; Acts 1:14; Galatians 1:19, were the cousins of the Lord, the sons of Alphaeus, Joseph's brother, and of Mary, the wife of Alphaeus, the sister-in-law (not sister) of the mother of the Lord. Others have held that they were the stepbrothers of Jesus, by a former marriage of Joseph. As a matter of fact, the question is of little import and can have no doctrinal significance. It is not for historical, exegetical, or dogmatic reasons, but only for motives of relevance that men have been prompted to insist upon the alleged fact of Mary's perpetual virginity.

The evangelist concludes the narrative by stating that he, Joseph, called the name of Mary's son Jesus, thus following the divine command, assuming the legal paternity of the child, and incidentally expressing his hopeful belief in the Savior of mankind.

Summary. Jesus Christ, the son and legal heir of David, beyond whom His genealogy can to Abraham, the father of the faithful of all times, was conceived and born of Mary, the virgin mother, after Joseph, His foster-father, had been instructed through a wonderful angelic vision as to God's interposition.