Christmas meant that Joseph practiced a rare combination: both righteousness and mercifulness (1:19)
The text says Joseph was “righteous.” Now this translates a Greek term (dikaios) which translates a Hebrew term (tsadiq) – and all these terms point to one thing: Joseph was known for doing whatever the Torah said. To do it, he had to know it. So, he was a man who had studied the Torah – either by listening and memorizing or by reading and memorizing. But, he knew it.
And that means he knew that he could take Mary to court for what was now known: she was pregnant, and Joseph knows that he is not the father. He immediately thought of the laws in Deuteronomy 22 – stone both the seducer and the one seduced or stone the rapist – the laws are clear and they are unavoidably clear. Here’s the text and its rulings that he would have known — and I’m asking that you read this carefully because following this text is exactly what Joseph would have equated with doing God’s will.

Deut. 22:23 If there is a young woman, a virgin already engaged to be married, and a man meets her in the town and lies with her, 24 you shall bring both of them to the gate of that town and stone them to death, the young woman because she did not cry for help in the town and the man because he violated his neighbor’s wife. So you shall purge the evil from your midst.
Deut. 22:25 But if the man meets the engaged woman in the open country, and the man seizes her and lies with her, then only the man who lay with her shall die. 26 You shall do nothing to the young woman; the young woman has not committed an offense punishable by death, because this case is like that of someone who attacks and murders a neighbor. 27 Since he found her in the open country, the engaged woman may have cried for help, but there was no one to rescue her.
Deut. 22:28 If a man meets a virgin who is not engaged, and seizes her and lies with her, and they are caught in the act, 29 the man who lay with her shall give fifty shekels of silver to the young woman’s father, and she shall become his wife. Because he violated her he shall not be permitted to divorce her as long as he lives.
Deut. 22:30 A man shall not marry his father’s wife, thereby violating his father’s rights.

Joseph is a tsadiq (I make much of this in Jesus Creed). This means his reputation is at stake: if he follows the Torah, and puts Mary away, he will uphold his reputation. If he does not, he will lose his reputation. (I like to say that the claims of the Cross were faced by Joseph [and Mary] before Jesus was born.)
Joseph, however, chooses another option – an option that must have set the agenda for Jesus learning how to respond to those stuck on the horns of a legal ruling that called for mercy. Joseph decides to put her away quietly – that is, instead of disgracing her (which he could have done) he chooses to be kind to her by divorcing her without publicly disgracing her.
Joseph must have impressed the same upon Jesus and the rest of his family: act in mercy; avoid humiliating others; you never know, you may be wrong in your discernments and judgments. [I wish I had some of this in Jesus Creed.]