On today's Gospel, Saint Jerome gives us the following words in the Office today at Matins.
Why must she who conceives the Lord be not simply a virgin, but a betrothed virgin? First, that, through the genealogy of Joseph the Davidic origin of Mary may be demonstrated. Second, that she may not be stoned as an adulteress by the Jews. Third, that she may have a protector during the flight into Egypt. The Martyr Ignatius adds a fourth reason for our Lord's being conceived by one who is betrothed: that His birth may be hidden from the devil, who thinks that this is the child of a married woman, not of a virgin.
"Before they came together, she was found with child of the Holy Ghost." Her condition was discovered by no one else but Joseph; concerning his future wife, he had almost the privilege of a husband to know everything about her. The qualification "before they came together" does not imply that afterwards they did come together. The Scripture is merely indicating that up to this time they had not done so.
"Joseph her husband, being a just man, and not willing to make her a publick example, was minded to put her away privily." If anyone is joined to a fornicator, he becomes one body with her; and it is a precept of the Law that not only the one who commits a crime, but anyone who is silently aware of it, is guilty of sin. Then how can Joseph be called a just man, when he is hiding his wife's crime? The question is not to the point. The point is that Joseph was a just man, and his conduct becomes a piece of evidence in Mary's favor. What he knew was not her crime (there was none to be known), but her chastity. What he did not know was the mystery of how she had conceived; and by his silence he kept hidden from the public the circumstance that was a source of wonder to him.